CHAPTER XIII

CLASS CEPHALOPODA

The Cephalopoda present a complete contrast to the majority of the Mollusca in habits and in many points of organisation. In their power of rapid movement and their means of progression, their extreme ferocity and carnivorous habits, their loss, in so many cases, of a shell, and in its constitution when present, in the general symmetry of their parts, in their reproductive and nervous system, they stand in a position of extreme isolation with nothing to connect them with the rest of the phylum.

Professor A. E. Verrill has collected many interesting details with regard to gigantic Cephalopoda occurring on the north-eastern coasts of America. From these it appears that the tentacular arms of some species of _Architeuthis_ measure as much as 32, 33, 35, and 42 feet in length, while the total length, including the body, sometimes exceeds 50 feet. Even off the Irish coast a specimen was once captured whose tentacular arms were 30 feet long, the mandibles 4 inches across, and the eyes about 15 inches in diameter.[393] The strength of these giant Cephalopods, aided as they are by formidable rows of suckers and other means of securing a grip, is almost incredible. Cases are not uncommon, in which persons diving or bathing have been attacked, and have with difficulty made their escape.

Great damage is frequently inflicted by Cephalopoda upon shoals of fish on British coasts. Off Lybster (Caithness) _Loligo_ and _Ommastrephes_ devour the herring, large numbers of which are cut up and bitten on the back of the neck by these creatures. On the American coasts the mackerel fisheries are sometimes entirely spoiled by the immense schools of squid which infest the Bay of St. Lawrence.[394] When excited in the pursuit of fish Cephalopoda leap high out of the sea. Dr. W. H. Rush[395] relates that when about 300 miles off the coast of Brazil, a swarm of hundreds of decapods flew from the water and landed on the deck of the ship, which was 12 feet above the surface level, and they had to go over the hammock nettings to reach it.

[Illustration: FIG. 238.--_Octopus vulgaris_ Lam., Naples: =A=, At rest; =B=, in motion; _f_, funnel, the arrow showing the direction of the propelling current of water. (After Merculiano.)]

The common _Octopus vulgaris_ Lam., of British and south European coasts, inhabits some rocky hole, the approaches to which, like the den of a fabled giant, are strewn with the bones of his victims. Homer himself knew how hard it is to drag the polypus out of his hole, and how the stones cling fast to his suckers. The colour-changes, which flit across the skin of the _Octopus_, appear, to some extent, expressive of the different emotions of the animal. They are also undoubtedly protective, enabling it to assimilate itself in colour to its environment. Mr. J. Hornell[396] has noticed an _Octopus_, while crawling over the rock-work in his tank, suddenly change the colour of the whole right or left side of its body, and of the four arms on the same side, to a snowy whiteness. They have also been seen to change colour, as if involuntarily, according to the material on which they crawl. The nerve-centres which control the chromatophores or pigment cells, causing them to expand or contract, are found to connect with the optic ganglia; hence the changes of colour may be regarded as a reflex result of the creature’s visual perception of its surroundings.

=Order Dibranchiata.=

Cephalopoda with two symmetrical branchiae, funnel completely tubular, mouth surrounded by 8 or 10 arms furnished with suckers or hooks, ink-sac and fins usually present, eyes with a lens; shell internal or absent.

The Dibranchiata are not known from Palaeozoic strata, and first appear (_Belemnites_, _Belemnoteuthis_) in the Trias. Whether they are to be regarded as derived from some form of Tetrabranchiata, _e.g._ _Orthoceras_, or as possessing an independent origin from some common stock, cannot at present be decided. They attain their highest development at the present time. The earliest representatives of the Order (the _Phragmophora_) possessed a shell chambered like that of the Tetrabranchiata. These chambered Dibranchiates rapidly reached their maximum in the upper Lias and as rapidly declined, until at the close of the Cretaceous epoch they were comparatively scarce, only a few genera (_Beloptera_, _Spirulirostra_) surviving into Tertiary times.

The ordinary Dibranchiate Cephalopod may be regarded as consisting of two parts--(_a_) the head, in which are situated the organs of sense, and to which are appended the prehensile organs and the principal organs of locomotion; (_b_) a trunk or visceral sac, enclosed in a muscular mantle and containing the respiratory, generative, and digestive organs. The visceral sac is often strengthened, and the viscera protected, by an internal non-spiral shell. The ‘arms’ which surround the mouth are modifications of the molluscan foot (p. 200), and are either eight or ten in number. In the former case (Octopoda) the arms, which are termed ‘sessile,’ are all of similar formation, in the latter (Decapoda), besides the eight sessile arms there are two much longer ‘tentacular’ arms, which widen at their tips into ‘clubs’ covered with suckers.

Remarks have already been made on the generative organs of Cephalopoda (p. 136 f.), the branchiae (p. 170), the nervous system (p. 206), the eye (p. 182), the radula (p. 236), and the ink-sac (p. 241).

One of the most characteristic features of the Dibranchiata are the _acetabula_, or suckers, with which the arms are furnished. They are usually disposed on the sessile arms in rows (of which there are four in most _Sepia_, two in _Octopus_, and one in _Eledone_), and become more numerous and smaller at the tip of the arm. They are massed together in large numbers of unequal size on the ‘clubs’ in the Decapoda, particularly in _Loligo_. In most Octopoda their base is flush with the surface of the arm, but in Decapoda the acetabula are pedunculate, or raised on short stalks. In Octopoda again, the acetabula are fleshy throughout, but in the Decapoda they are strengthened by a corneous rim with a smooth or denticulate edge (_Ommastrephes_, _Architeuthis_). Many of the acetabula on the tentacular and sometimes on the sessile arms of the Onychoteuthidae enclose a powerful hook, which is retractile like the claws of a cat.

In mechanical structure the acetabula consist of a disc with a slightly swollen margin, from which a series of muscular folds converge towards the centre of the disc, where a round aperture leads to a gradually widening cavity. Within this cavity is a sort of button, the _caruncle_, which can be elevated or depressed like the piston of a syringe; thus when the sucker is applied the piston is withdrawn and a vacuum created (Owen).

[Illustration: FIG. 239.--‘Club’ of _Loligo vulgaris_ L., showing the crowded pedunculate acetabula, × ½.]

[Illustration: FIG. 240.--One of the suckers of _Architeuthis dux_ Stp., showing the denticulate margin and corneous ring; _p_, peduncle.]

In many Octopoda the arms are connected by a web (the _umbrella_), which sometimes extends up the greater part of the arms (_Cirrhoteuthis_, some _Eledone_), at others occurs only at the base. The use of the umbrella is perhaps to assist in locomotion, by alternate contraction and expansion.

A cartilaginous skeleton is well developed, especially in the Decapoda. In _Sepia_ a cephalic cartilage forms a complete ring round the oesophagus, the eyes being situated in lateral prolongations of the same. In front of the cephalic cartilage occurs a piece like an inverted T, which supports the base of the anterior arms. The Decapoda have also a ‘nuchal’ cartilage, connecting the head with the anterior dorsal portion of the mantle, while cartilaginous knobs on the ventral mantle button into corresponding sockets on the funnel.

=Sub-order I.=--_Octopoda._--Body round or bag-like, generally without fins, arms eight, suckers fleshy, usually sessile, oviducts paired, no nidamental glands, shell absent.

[Illustration: FIG. 241.--_Cirrhoteuthis magna_ Hoyle, S. Atlantic. Two of the left arms and their web have been removed: _f_, funnel; _fi_, _fi_, fins; _m_, mouth. (After Hoyle, × 1/12.)]

FAM. 1. _Cirrhoteuthidae._--Body with two prominent fins; arms in great part united by a web; one row of small suckers, with cirrhi on each side.--Atlantic and Pacific Oceans, deep water (Fig. 241).

FAM. 2. _Amphitretidae._--Body gelatinous, mantle fused with the funnel in the median line, forming two openings into the branchial cavity; arms with one row of suckers; umbrella extending more than two-thirds up the arms.--South Pacific (Fig. 242).

The two pocket-like openings into the branchial cavity are unique among Cephalopoda (Hoyle).

[Illustration: FIG. 242.--_Amphitretus pelagicus_ Hoyle, off Kermadec Is.: _e_, eyes; _f_, funnel; _p_, right mantle-pocket. (After Hoyle.)]

FAM. 3. _Argonautidae._--Female furnished with a symmetrical, unilocular shell, spiral in one plane, secreted by thin terminal expansions (the _vela_) of the two dorsal arms, no attachment muscle; suckers in two rows, pedunculate; male very small, without veligerous arms or shell.--All warm seas (Fig. 243). Pliocene----.

The shell consists of three layers, the two external being prismatic, the middle fibrous. Its secretion by the arms and not by the mantle edge is unique, and shows that it is not homologous with the ordinary molluscan shell.

[Illustration: FIG. 243.--_Argonauta argo_ L., the position assumed by a specimen kept in captivity, the arrow showing the direction of movement: _f_, funnel; _m_, mouth, with jaws projecting; _sh_, shell, with arms as seen through it; _wa_, webbed arm clasping shell. (After Lacaze-Duthiers.)]

The great controversy on the _Argonauta_, which once raged with so much fierceness, is now matter of ancient history. It seems scarcely credible that between fifty and sixty years ago, two of the leading zoologists of the day, Mr. Gray and M. de Blainville maintained that the animal which inhabits the Argonaut shell is a parasite, without any means of depositing or forming a shell of its own, but which possesses itself of the Argonaut shell, either by expelling or succeeding the original inhabitant, a supposed nucleo-branchiate (Heteropod) mollusc akin to _Carinaria_. The final blow to this strange hypothesis--which was urged by the most ingenious series of arguments--was given by Professor Owen, who in 1839 brought before the Zoological Society of London the admirable observations of Madame Jeannette Power, who made a continuous study of a number of specimens of _Argonauta_ in her vivarium at Messina. The result of these observations tended to show that the young _Argonauta_ when first excluded from the egg is naked, but that in ten or twelve days the shell begins to form, that the principal agents in the deposition of shell are the two velated or web-like arms; and that portions of the shell, if broken away, are repaired by a deposition of calcareous matter.[397]

FAM. 4. _Philonexidae._--Mantle supported by two ridges placed on the funnel; large ‘aquiferous’ pores (supposed to introduce water into the tissues) near the head or funnel; suckers in two rows, pedunculate.--Atlantic and Mediterranean.

Genera: _Ocythoe_, arms of unequal size, no intervening membrane, third arm on the right hectocotylised (see Fig. 51, p. 138), two aquiferous pores at the base of the siphon; male very small; _Tremoctopus_, two aquiferous pores between the eyes, two on the ventral side of the head.

FAM. 5. _Alloposidae._--Mantle edge united to the head by three commissures; arms extensively webbed, acetabula sessile. Hectocotylised arm developed in a cavity in front of the right eye.--N. Atlantic.

FAM. 6. _Octopodidae._--Head very large, arms elongated, similar, more or less webbed, acetabula usually in two rows, sessile; mantle supported by fleshy bands, no cephalic aquiferous pores.

In _Octopus_ proper the web is usually confined to the lower part of the arms; Fischer separates off as _Pteroctopus_ a form in which it reaches almost to their extremity. The third right arm (Fig. 52, p. 140) is hectocotylised, the modified extremity being, according to Hoyle, sometimes minute, sometimes spoon-shaped, with a tendency to transverse ridges, rarely slender and very long. The relative length of the pairs of arms varies in different species. Two cartilaginous stylets, imbedded in the dorsal mantle, are said by Owen to represent the shell.

Other genera; _Pinnoctopus_, body furnished with broad lateral wings which meet at the posterior end; _Cistopus_, a large web prolonged along the sides of the arms, fitted with oval aquiferous pouches, with pores at their base, between each pair of arms; _Eledone_ (Fig. 244), one row of acetabula; _Tritaxeopus_, _Iapetella_.

=Sub-order II.=--_Decapoda._--Body oblong, mouth surrounded by four pairs of sessile and one pair of tentacular arms, the latter terminated by a ‘club’; acetabula pedunculate and furnished with a corneous margin; mantle margin locked to the base of the funnel by a cartilaginous apparatus; head and anterior part of body furnished with aquiferous pores; fins present; mandibles corneous; oviduct single, large nidamental glands in the female; shell internal.

[Illustration: FIG. 244.--_Eledone Aldrovandi_ Delle Chiaje, Naples, from ventral side, × ½.]

The tentacular arms, which are the principal external feature of the Decapoda, are not derived from the same muscular ring as the sessile arms, but arise from the cephalic cartilage, and emerge between the third and fourth arm on each side. In _Sepia_ they can be entirely retracted into a kind of pocket behind the eyes, while in _Loligo_ they are simply folded over one another. In _Chiroteuthis_ the arms are six times as long as the body, and the clubs have four rows of denticulate suckers.

The anterior ventral[398] portion of the mantle is furnished with a singular contrivance for locking it to the funnel, and so rendering the whole animal more capable of resisting the impact of any force. This contrivance generally consists of a series of ridges or buttons which fit into grooves or button-holes, the ridges being on the interior face of the mantle and the corresponding grooves on the funnel, or _vice versâ_. The ‘resisting apparatus’ is most elaborate in the pelagic genera, and least so in the more sluggish littoral forms. A similar, but not so complex, arrangement occurs also in the Octopoda.

The different forms of the shell appear to indicate successive stages in a regular course of development. We have in _Spirula_ (Fig. 247) a chambered shell of the Tetrabranchiate type, but of considerably diminished size, which has ceased to contain the animal in its last chamber, and has become almost entirely enveloped in reflected folds of the mantle. These folds gradually concresce to form a definite shell-sac, by the walls of which are secreted additional laminae of calcareous shell-substance. These laminae invest the original shell, which gradually (_Spirulirostra_, _Belosepia_) loses the spiral form and becomes straight, eventually disappearing, while the calcareous laminae alone remain (_Sepia_). These in their turn disappear, leaving only the plate or ‘pen’ upon which they were deposited (_Loligo_), which itself also, with the shell-sac, finally disappears, surviving only in the early stages of _Octopus_ (Lankester).

The Decapoda are divided, according to the character of the shell, into _Phragmophora_, _Sepiophora_, and _Chondrophora_.[399]

[Illustration: FIG. 245.--‘Club’ of _Onychoteuthis_ sp., showing the hooks and clusters of fixing cushions and acetabula below them, × ½.]

A. PHRAGMOPHORA.--Arms furnished with hooks or acetabula; shell consisting of a _phragmocone_ or chambered sac enclosed in a thin wall (the _conotheca_), septa pierced by a siphuncle near the ventral margin (in _Spirula_ alone this chambered sac forms the whole of the shell). The apex of the cone lies towards the posterior end of the body, and is usually enveloped in a calcareous _guard_ or _rostrum_. Beyond the anterior end of the rostrum the conotheca is extended forward dorsally by a _pro-ostracum_ or anterior shell, which may be shelly or horny, and corresponds to the _gladius_ of the Chondrophora. The rostrum consists of calcareous fibres arranged perpendicularly to the planes of the laminae of growth, and radiating from an axis, the so-called _apical line_, which extends from the extremity of the phragmocone to that of the rostrum. _Distribution_, see p. 380.

FAM. 1. _Spirulidae._--Arms with acetabula, shell a loose spiral, without rostrum or pro-ostracum, partially external, enclosed in two lobes of the mantle (Figs. 247 and 248).

The single species of the single genus (_S. Peronii_ Lam. = _laevis_ Gray) has not yet been thoroughly investigated, although the shell occurs in thousands on many tropical beaches, and is sometimes drifted on our own shores. The animal appears to have the power of adhesion to the rocks by means of a terminal sucker or pore. The protoconch is present, and contains a prosiphon, which does not connect with the siphuncle. The septal necks are continuous, not broken as in _Nautilus_. The siphuncle is on the ventral margin of the shell, the last whorl of which projects slightly on the dorsal and ventral sides, but is even there covered by a thin fold of the mantle. The retractor muscles of the funnel and of the head find their _point d’appui_ on the shell, the last chamber of which contains the posterior part of the liver, with which the membranous siphuncle is connected.

FAM. 2. _Belemnitidae._--Arms hooked as in _Onychoteuthis_, fins large; phragmocone straight, initial chamber globular, larger than the second, rostrum often very long, investing the phragmocone, pro-ostracum sword- or leaf-shaped, rounded in front, seldom preserved, ink-sac present.--Lower Lias to Cretaceous.

[Illustration: FIG. 246.--_Sepia officinalis_ L., with mantle cut away to show position of internal shell, × ½. (The ends of the tentacular arms are cut off.)]

The Belemnitidae are believed to have been gregarious, and to have lived in shallow water on a muddy bottom. Specimens are sometimes found in which even the ink-sac can be recognised _in situ_. The relative proportions of rostrum and phragmocone vary greatly in different groups, the rostrum being in some cases two feet long, in others only just enclosing the phragmocone. As a rule the rostrum is the only portion which has been preserved.

FAM. 3. _Belosepiidae._--Phragmocone short, slightly curved, chambers small, placed at the posterior end of a sepion, rostrum solid, obtuse.--Eocene (Paris, Bracklesham, etc.).

_Fam. 4._ _Belopteridae._--Sepion not known; phragmocone curved, siphuncle on the ventral margin, rostrum well developed, pointed. Principal genus, _Spirulirostra_.--Miocene of Turin.

These two families, with their small, curved phragmocone and (in the case of the Belosepiidae) large sepion, are clearly intermediate between the Phragmophora and Sepiophora. Some authorities place them with the latter group.

[Illustration: FIG. 247.--Shell of _Spirula Peronii_ Lam. =A=, Cutside view; =B=, showing last chamber and position of siphuncle; =C=, in section, showing the septa and course of siphuncle; =D=, shell broken to show the convexity of the inner side of the septa; =E=, portion of a septal neck.]

[Illustration: FIG. 248.--_Spirula Peronii_ Lam.: _d_, terminal sucker; _f_, funnel; _s_{1}_, _s_{2}_, projecting portions of shell, the internal part of which is dotted in. (From Owen and A. Adams combined.)]

B. SEPIOPHORA.--Shell internal, consisting usually of (_a_) an anterior cancellated portion, (_b_) a posterior laminated portion, the laminae enclosing air. It terminates in a very rudimentary phragmocone and a rostrum, but there is no siphuncle.

FAM. _Sepiidae._--Eyes with cornea complete, body oval, fins narrow, lateral, as long as the body, generally united behind; sessile arms short, tentacular arms long, acetabula generally in four rows, fourth left arm in the male hectocotylised near the base (Fig. 249).--World-wide.

The sepion or ‘cuttle-bone’ runs the whole length and width of the body. In _Sepia_ it is very thick in front, while the posterior ventral end is concave and terminated by a prominent spine, the _rostrum_ or _mucro_ which points downwards. The whole shell is surrounded by a thin chitinous margin, which forms a lateral expansion. Other genera are _Sepiella_, _Hemisepius_, and _Trachyteuthis_ (fossil only).

C. CHONDROPHORA.--Shell (_gladius_ or _pen_) long, chitinous.

(_a_) _Myopsidae:[400] cornea entire, species mostly sub-littoral._

FAM. 1. _Sepiolidae._--Fins large, dorso-lateral; tentacular arms retractile; two first dorsal arms in the male hectocotylised; gladius narrow, half as long as the body.--World-wide.

Principal genera: _Sepiola_, dorsal mantle connected with the head by a broad cervical band, ventral mantle with the funnel by a ridge fitting into a groove; _Rossia_, dorsal mantle supported by a ridge, arms with never more than four rows of acetabula; _Inioteuthis_, _Stoloteuthis_, _Nectoteuthis_, and _Promachoteuthis_.

FAM. 2. _Sepiadariidae._--Fins not as long as the body, mantle united to the head on the dorsal side, fourth left arm in the male hectocotylised; no gladius. Principal genera, _Sepiadarium_, _Sepioloidea_.--Chiefly Pacific Ocean.

[Illustration: FIG. 249.--Hectocotylised arm (_h.a._) of _Sepia officinalis_ L., shown in contrast to one of the ordinary sessile arms; _m_, mouth; _p_, pocket into which the tentacular arm is retracted.]

FAM. 3. _Idiosepiidae._--Fins very small, terminal; fourth pair of arms in the male hectocotylised, bare of suckers.

The only genus, _Idiosepion_, with a single species (_I. pygmaeum_ Stp.) is from the Indian Ocean, and is the smallest known Cephalopod, measuring only about 15 mm. in length.

FAM. 4. _Loliginidae._--Body rather long, fins varying in size, tentacular arms partially retractile, gladius as long as the back, pointed in front, shaft keeled on the ventral side.--World-wide.

_Loligo_ proper has a pointed body with triangular posterior fins united behind; sessile arms with two rows of acetabula, tentacular arms with four; fourth left arm hectocotylised at the tip; funnel attached to the head. Other genera are _Loliguncula_, _Sepioteuthis_, and _Loliolus_. _Belemnosepia_, _Beloteuthis_, _Leptoteuthis_, and _Phylloteuthis_ are fossil genera only, differing in the shape of the gladius.

(_b_) _Oigopsidae: cornea more or less open; species pelagic._

FAM. 5. _Ommastrephidae._--Body cylindrical, fins generally terminal, united together, regularly rhomboidal, sessile arms with varying number of rows of acetabula, mantle connexions elaborate; gladius horny, narrow lanceolate, with a hollow cone at the posterior end.--World-wide.

[Illustration: FIG. 250.--_Architeuthis princeps_, Verr., E. America: _f_, Right fin; _fu_, funnel; _f.c_, fixing cushions and acetabula on the tentacular arms (_t_, _t_). (After Verrill, × 1/60.)]

_Ommastrephes_ proper has a natatory web on the sessile arms; the wrist of each club has a series of acetabula with corresponding cushions on the other wrist. In _Thysanoteuthis_ (often made a separate family) the sessile arms have two rows of cirrhi, with lateral expansions of the skin; fins as long as the body. In _Architeuthis_, to which belong the largest Cephalopoda known, the fins together are shaped like a broad arrow-head; acetabula of sessile arms strongly denticulate; tentacular arms very long, with equidistant pairs of acetabula and fixing cushions throughout their entire length, and a group of the same at the base of the club. The acetabula and cushions correspond on the opposing tentacles, and enable them to pull together. Other genera are _Dosidicus_, _Todarodes_, _Illex_, _Bathyteuthis_ and _Mastigoteuthis_.

FAM. 6. _Onychoteuthidae._--Body cylindrical, fins terminal or lateral, mantle-locking apparatus elaborate, tentacular arms very long, sessile or tentacular arms furnished with retractile hooks, gladius lanceolate, with a terminal cone.--World-wide.

The prehensile apparatus of Cephalopoda reaches its maximum of power and singularity in this family. In _Onychia_, _Onychoteuthis_ and _Ancistroteuthis_, the sessile arms have acetabula only, in _Gonatus_ and _Abralia_ they have hooks as well, while in _Verania_, _Ancistrochirus_ and _Enoploteuthis_, the sessile arms have hooks only. The number of rows of hooks or acetabula varies with the different genera.

FAM. 7. _Chiroteuthidae._--Head nearly as large as the body; fins terminal, tentacular arms very long, sessile arms slightly webbed, acetabula denticulated; mantle-supports consisting of cartilaginous ridges on the mantle, which fit into corresponding depressions on the funnel, gladius expanded at each end.--Atlantic Ocean.

The six dorsal arms in _Histioteuthis_ are united by a broad web, while in _Histiopsis_ the web only reaches half way up the arm. In _Chiroteuthis_ the tentacular arms have scattered sessile suckers throughout their whole length, and four rows of very long pedunculate suckers on the clubs.

FAM. 8. _Cranchiidae._--Head small, body rounded, barrel-shaped, fins terminal, eyes often very large, sessile arms short, tentacular arms long, thread-like.--World-wide.

_Cranchia_ proper has the tentacular clubs finned, with eight rows of suckers, body sometimes covered with warty tubercles. _Loligopsis_ has a very attenuated body, with fins terminally united; some species are spotted with colour, or have rows of tubercles on the ventral side. _Taonius_ (Fig. 251) is doubtfully distinct from _Loligopsis_.

[Illustration: FIG. 251.--_Taonius hyperboreus_ Stp., N. Atlantic: _e_, _e_, eyes; _f_, _f_, fins; _t_, _t_, tentacular arms. (After Hoyle, × ¼.)]

=Order Tetrabranchiata=

Cephalopoda with four branchiae and four kidneys; animal inhabiting the last chamber of an external multilocular shell; funnel consisting of two separate lobes; tentacles numerous, without suckers or hooks; no ink-sac.

The shell consists of two layers, the outer being porcellanous, and the inner, as well as the walls of the chambers or _septa_, nacreous. The septa vary greatly in shape. In most of the Nautiloidea they are regularly curved, as in _Nautilus_, or straight, as in _Orthoceras_, but in the Ammonoidea they are often exceedingly complex. The edge of the septum, where it unites with the shell-wall, is called the suture, and the sutural line, which is not seen until the porcellanous layer is removed, varies in shape with the septum.

[Illustration: FIG. 252.--_Nautilus pompilius_ L., in section, showing the septa (_s_, _s_), the septal necks (_s.n_, _s.n_), the siphuncle dotted in (_si_), and the large body chamber (_ch_).]

The septa are traversed by a membranous tube known as the _siphuncle_, which in _Nautilus_ is said by Owen to connect ultimately with the pericardium. The _septal necks_, or short tubular prolongations of the septa where they are perforated by the siphuncle, are in the great majority of the Nautiloidea directed backwards (Fig. 252), _i.e._, they project from the front wall of each chamber, while in nearly all Ammonoidea they are directed forwards. When the siphuncle is narrow, as in the Ammonoidea, it is simple, but when wide, as in many of the Nautiloidea, its walls are often thickened by the deposition of masses of calcareous matter, or by rings and radiating lamellae of the same material. In position, the siphuncle is sometimes central, sometimes sub-central, sometimes (Ammonoidea) marginal. In some cases its position is believed to change during the growth of the individual. The precise object served by the siphuncle is at present unknown. Some hold that it preserves the vitality of the unoccupied chambers, by connecting them with the soft parts of the animal; others have regarded it as a means for lightening the shell by the passage of some gas into the chambers.

[Illustration: FIG. 253.--_Ammonites_ (_Cadoceras_) _sublaevis_ Sowb., Kellaway’s Rock, showing the marginal position of the siphuncle (_si_).]

The initial chamber in Nautiloidea consists of an obtuse incurved cone, marked on the outer surface of its posterior wall by a small scar known as the _cicatrix_, which may be slit-like, round, oval, or cruciform in shape. It has been held that the cicatrix originally communicated with the protoconch or larval shell, which probably dropped off as development proceeded. In the Ammonoidea, on the other hand, there is no cicatrix, and the initial chamber probably represents the protoconch, as seen in the nucleus of many Gasteropoda.

=Sub-order 1.= _Nautiloidea._--Shell straight, bent, or coiled, aperture simple or contracted; siphuncle often narrowed by internal deposits, position variable; septal necks short, usually directed backwards; septa concave towards the aperture; initial chamber conical, with a cicatrix on the posterior wall.

The Nautiloidea, of which _Nautilus_ is the sole living representative, date back to the Cambrian epoch, and attain their maximum in the Silurian and Devonian. At the close of the Palaeozoic era, every family, with the sole exceptions of the _Orthoceratidae_ and _Nautilidae_, appears to have become extinct. The former disappear with the Trias, and after the lapse of the whole Secondary era, _Aturia_, a form closely related to _Nautilus_, makes its appearance.

(_a_) _Retrosiphonata_: septal necks directed _backwards_.

FAM. 1. _Orthoceratidae._[401]--Shell straight or slightly curved, aperture simple, body-chamber large; siphuncle cylindrical, position variable. Single genus, _Orthoceras_ (Fig. 254). Cambrian to Trias.

FAM. 2. _Endoceratidae._--Shell straight, siphuncle wide, marginal, septal necks produced into tubes fitting into one another. Principal genera: _Endoceras_ (specimens of which occur six feet long), and _Piloceras_--Ordovician.

FAM. 3. _Actinoceratidae._--Shell straight or slightly curved, siphuncle wide, contracted at the septa by obstruction-rings. Principal genera: _Actinoceras_, _Discosorus_, _Huronia_, _Sactoceras_.--Ordovician to Carboniferous.

FAM. 4. _Gomphoceratidae._--Shell globular, straight or considerably curved, aperture narrowed, T-shaped, body-chamber large, siphuncle variable in position. The aperture is in some cases so narrow that probably only the arms could be protruded. Principal genus, _Gomphoceras_ (Fig. 255).--Silurian.

FAM. 5. _Ascoceratidae._--Shell sac-like or flask-shaped, apex truncated, unknown, body-chamber occupying nearly the whole of the shell on the ventral side, contracting at the aperture, last few septa coalescing on the dorsal side and encroaching upon the body-chamber. The young form has a symmetrical shell like _Orthoceras_, attached to the sac-like shell above described; as growth proceeds the former portion is thrown off. Principal genera: _Ascoceras_, _Glossoceras_.--Ordovician and Silurian.

[Illustration: FIG. 254.--=A=, Section of _Orthoceras_, showing the septa (_s_, _s_), and siphuncle (_si_, _si_); =B=, portion of the exterior of _Orthoceras annulatum_ Sowb., × ½. (Woodwardian Museum, Cambridge.)]

FAM. 6. _Poterioceratidae._--Shell fusiform, contracted at both ends, aperture simple, siphuncle variable in position, inflated between the septa. The form generally resembles _Gomphoceras_, except for the simple aperture and fusiform shape.--Ordovician to Carboniferous.

FAM. 7. _Cyrtoceratidae._--Shell conical or sub-cylindrical, slightly curved, body-chamber large, siphuncle variable in position. Single genus, _Cyrtoceras_.--Cambrian to Carboniferous.

FAM. 8. _Lituitidae._--Shell coiled in a flat, sometimes loose spiral, last whorl straight, containing the body-chamber, often greatly prolonged. Principal genera: _Lituites_, _Ophidioceras_.--Ordovician and Silurian.

FAM. 9. _Trochoceratidae._--Shell helicoid, with seldom more than two whorls, dextral or sinistral, last whorl sometimes partly uncoiled. Principal genera: _Trochoceras_, _Adelphoceras_.--Ordovician to Devonian.

FAM. 10. _Nautilidae._--Shell with few whorls, more or less overlapping, septa simple, siphuncle central or sub-central, aperture not contracted.

The ‘tentacles’ are about 90 in number, and consist of four groups each of 12 or 13 labial tentacles surrounding the mouth, two groups each of 17 larger (brachial) tentacles on each side of the head, two thicker tentacles which combine to form the ‘hood,’ and two small tentacles on each side of the eye. When the animal swims, the tentacles are extended radially from the head, somewhat like those of a sea-anemone. The direction of the many pairs of tentacles at constant but different angles from the head, is the most striking feature in the living _Nautilus_, and accounts for its being described, when seen on the surface, as ‘a shell with something like a cauliflower sticking out of it.’[402] The funnel is not a complete tube, but is formed by the overlapping of the margins of two thin fleshy lobes (which are probably morphologically epipodia), so that when the two lobes are parted, a broad canal appears, leading to the branchial cavity. The head is conical, and the mouth and its appendages can be retracted into a sort of sheath, over which fits the ‘hood.’

[Illustration: FIG. 255.--=A=, _Gomphoceras ellipticum_ M’Coy, Silurian: =B=, aperture (_ap_) of same; _s_, _s_, septa; _si_, position of siphuncle. (After Blake.)]

Other genera are _Trocholites_, _Gyroceras_, _Hercoceras_, _Discites_, _Aturia_.--Ordovician to present time.

FAM. 11. _Bactritidae._--Shell straight, conical, siphuncle small, marginal, septal necks long, funnel-shaped, sutures undulating, with a sinus corresponding to the siphuncle. This family, from the form of its sutures, appears to constitute a passage to the Ammonoidea. Single genus, _Bactrites_.--Silurian and Devonian.

(_b_) _Prosiphonata._--Septal necks directed _forwards_.

The two genera are _Bathmoceras_ (Ordovician), shell straight, conical always truncated, siphon marginal; and _Nothoceras_ (Silurian), shell nautiloid with simple sutures.

=Sub-order 2.= _Ammonoidea._--Shell multiform, straight, curved, flat spiral, or turreted, sutural line more or less complex, siphuncle simple.

Some authorities hold that the members of this great sub-order, now totally extinct, belong to the Dibranchiata, on the ground that the protoconch resembles that of _Spirula_ rather than that of the Nautiloidea. Others again regard the Ammonoidea as a third, and distinct Order of Cephalopoda. Their distribution extends from the Silurian to (possibly) the early Tertiary. No trace has ever been found of an ink-sac, mandible, or hooks on the arms; the shell was undoubtedly external.

[Illustration: FIG. 256.--Diagram of the sutures of Ammonites: =A=, an elaborate suture (_Phylloceras_); =B=, a simple suture (_Ceratites_); _s.s_, siphonal, _s.v_, ventral, _s.l_, first lateral, _s.l´_, second lateral saddles; _s.a_, _s.a_, auxiliary saddles; _l.v_, ventral, _l_, first lateral, _l´_, second lateral lobe; _l.a_, _l.a_, auxiliary lobes. The arrow points _towards_ the aperture. (From Woodward.) Compare Fig. 258.]

The sutural line, which indicates the septa, and is generally concealed beneath the outer layer of shell, consists of a number of _lobes_ or depressions, the concave part of which is directed towards the aperture. Between these lobes lie corresponding elevations, or _saddles_, the convex part of which is directed towards the aperture. There are six principal lobes (Fig. 256): the _siphonal_ or _ventral_, which is traversed by the siphuncle, the _dorsal_, and a superior and inferior lateral on each side; smaller auxiliary lobes may succeed these latter. The adjacent saddles have received corresponding names. As a rule the sutural line is very complex, but in some cases (_Goniatites_, _Lobites_) it is simple (Fig. 258, A). The first saddle of a large number of genera serves as a means of classification, according as it is broad or narrow. Some authorities reverse the terms ventral and dorsal, as applied above. It is probable, however, that the position of the animal of _Ammonites_ in its shell resembled that of _Nautilus_. The siphuncle is dorsal (internal) in _Clymenia_ only, ventral (external) in all other genera.

The _aptychus_ of Ammonoidea is a corneous or calcareous valve-like body, generally formed of two symmetrical parts (Fig. 257). It has been regarded by some as the covering of the nidamental gland, and hence as occurring only in the female, by others, with more probability, as an operculum, covering or imbedded in a hood formed, as in _Nautilus_, of modified arms. Sometimes the Aptychus is in a single piece (_Anaptychus_), sometimes the two pieces are united on the median line (_Synaptychus_).

[Illustration: FIG. 257.--Aptychus of Ammonite (_Trigonellites latus_). Kimmeridge Clay, Ely. × ½.]

The Ammonoidea are thus classified by Dr. P. Fischer:--

(_a_) Retrosiphonata _Goniatitidae_.

{ First saddle, { _Arcestidae_, _Tropitidae_, { No Aptychus or { wide { _Ceratitidae_, _Clydonitidae_. { Anaptychus { { corneous, { { _Pinacoceratidae_, _Amaltheidae_, (_b_) Prosiphonata { single { First saddle, { _Ammonitidae_, { { narrow { _Lytoceratidae_. { { Aptychus calcareous, valves { _Harpoceratidae_, _Stephanoceratidae_. { double or united

(_a_) _Retrosiphonata._ FAM. 1. _Goniatitidae._--Shell nautiloid, whorls sometimes disjoined, siphuncle ventral or dorsal, sutures simple. Principal genera: _Clymenia_, _Goniatites_ (Fig. 258, A).--Devonian to Carboniferous.

(_b_) _Prosiphonata._ FAM. 2. _Arcestidae._--Shell globular, smooth or striated and rayed, body-chamber very long, aperture often with a projecting hood, umbilicus closed by a callosity, lobes numerous, foliaceous, aptychus present. Principal genera: _Arcestes_, _Lobites_.--Principally Trias.

FAM. 3. _Tropitidae._--Differs from Arcestidae mainly in the more highly ornamented surface, which is decorated with ribs which become granular at the periphery. Principal genus, _Tropites_.--Trias and Lias.

FAM. 4. _Ceratitidae._--Shell ribbed and tuberculated, body-chamber short, lobes denticulated, saddles simple. Principal genera: _Ceratites_ (Fig. 258, B), _Trachyceras_.--Principally Trias.

FAM. 5. _Clydonitidae._--Shell variable in form, body-chamber short, sutural line undulated, simple. Principal genera: _Clydonites_, _Choristoceras_, _Rhabdoceras_, _Cochloceras_.--Trias.

FAM. 6. _Pinacoceratidae._--Shell discoidal, usually smooth, body-chamber short, sutural line very complex, lobes numerous. Principal genera: _Pinacoceras_, _Sageceras_.--Carboniferous to Trias.

FAM. 7. _Amaltheidae._--Shell broad, keeled, last whorl concealing most of the spire, sutures with auxiliary lobes, incised.--Principal genera: _Amaltheus_, _Schloenbacia_, _Sphenodiscus_.--Trias, Cretaceous.

[Illustration: FIG. 258.--Various forms of Ammonoidea: =A=, _Goniatites crenistria_ J. Phil., Carb. Limestone; =B=, _Ceratites nodosus_ de Hann., Muschelkalk; =C=, _Ammonites_ (_Parkinsonia_) _Parkinsoni_ Sowb., Inf. Oolite; =D=, _Phylloceras helerophyllum_ Sowb., Upper Lias; _s_, _s_, sutural lines.]

FAM. 8. _Ammonitidae._--Body-chamber long, whorls narrow, uncovered, more or less ribbed, aperture simple, sutural line normal, aptychus single, corneous. Principal genera: _Ammonites_, _Aegoceras_.--Principally Lias.

FAM. 9. _Lytoceratidae._--Shell discoidal, body-chamber short, aperture simple, no aptychus. Principal genera: _Lytoceras_, _Phylloceras_ (Fig. 258, D).--Trias to Cretaceous.

FAM. 10. _Harpoceratidae._--Shell discoidal, compressed, margin keeled, surface with straight or arched ribs, aperture with lateral projections, suture with accessory lobes, aptychus in two pieces. Principal genera: _Harpoceras_, _Oppelia_, _Lissoceras_.--Jurassic to Cretaceous.

[Illustration: FIG. 259.--=A=, _Turrilites catenulatus_ d’Orb, Gault; =B=, _Macroscaphites Iranii_ d’Orb, Upper Neocomian. (From Zittel.)]

FAM. 11. _Stephanoceratidae._--Shell discoidal, helicoid or straight, whorls sometimes disunited, surface often with bifurcating ribs, which are tubercled, aperture often with lateral projections, sutural line incised, aptychus in two pieces, sometimes united.

In the discoidal group, _Stephanoceras_ is strongly ribbed, tubercled at the point of bifurcation, _Cosmoceras_ has long lateral projections of the aperture when young, _Perisphinctes_ has a large body-chamber and numerous smooth ribs. Other genera are _Acanthoceras_, _Peltoceras_, _Aspidoceras_, and _Hoplites_. Among the loosely whorled genera, _Scaphites_ (Fig. 260, A) has the last whorl produced and bent back again in horse-shoe form, while the early whorls are concealed; _Hamites_, _Hamulina_, and _Ptychoceras_ have a shell shaped like a single or double hook, the sides of which may or may not be united; _Crioceras_ (Fig. 260, B) in form of whorls resembles a _Spirula_, _Ancyloceras_ a _Scaphites_ with the first whorls disunited. _Macroscaphites_ (Fig. 259, B) is similar, but with the first whorls united and not concealed. _Turrilites_ (Fig. 259, A) is turreted and sinistral, while _Baculites_ is quite straight, with a long body-chamber.

[Illustration: FIG. 260.--=A=, _Scaphites aequalis_ Sowb., Cretaceous; =B=, _Crioceras bifurcatum_ Quenst., Cretaceous. (From Zittel.)]

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