Part 16

_Phylogeny and Classification._--As _Nautilus_ is the only living genus of the Tetrabranchiata, our knowledge of all the rest is based upon the study of their fossil shells. A vast number of species of shell similar in structure to that of _Nautilus_ are known, chiefly from Primary and Secondary formations. These are divided into two sub-orders by differences in the form and structure of the initial chamber. In the Nautiloidea this chamber has the form of an obtuse cone, on the apex of which is a slit-like mark or cicatrix, elongated dorso-ventrally and placed opposite to the blind end of the siphuncle, which indents the front wall of the initial chamber but does not enter its cavity. In the Ammonoidea, on the other hand, the initial chamber is inflated, and is spheroidal, oval or pyriform in shape, with no cicatrix, and separated from the first air-chamber by a constriction. The siphuncle also commences with a dilatation which deeply indents the front wall of the initial chamber, called the protoconch, but does not penetrate into its cavity. Munier-Chalmas has shown that the cavity of the protoconch is traversed by a tubular organ, the "prosiphon," which does not communicate with the true siphuncle, the place of which it is supposed to take in the early life of the animal. It is generally held, as suggested by Alpheus Hyatt, that the initial chamber of the Nautiloidea corresponds not to the protoconch of the Ammonoids, but to the second chamber of the latter, and that there existed in the young Nautiloids a true initial chamber, a protoconch which was either uncalcified or deciduous. The shell of the living nautilus does not decide this question, as its early stages are unknown, and there is a little vacuity in the centre of the spirally coiled shell which may have been originally occupied by the true protoconch.

The septa in the Nautiloidea are generally concave towards the aperture of the shell, their curvature therefore directed backwards (fig. 1); in the Ammonoidea, on the other hand, the convexity is usually towards the aperture, the curvature therefore directed forwards. The lines along which the edges of the septa are united to the shell are known as "sutures," and these in the Nautiloidea are simply curved or slightly lobed, whereas in the Ammonoidea they are folded in various degrees of complexity; the projections of the suture towards the mouth of the shell are called saddles, those in the opposite direction lobes. The siphuncle in the _Nautilus_ pierces the centres of the septa, and in fossil Nautiloids it is usually central or sub-central. In a few cases it is marginal, and in that case may be external, i.e. ventral, or internal, i.e. dorsal. In Ammonoids the siphuncle is always marginal, and usually external. Its walls in the living _Nautilus_ are strengthened by the deposit of calcareous granules, and in some fossil forms the wall is completely calcified. But this proper calcified wall is quite distinct from calcareous tubes surrounding the siphuncle, which are developed from the septa. In the pearly nautilus each septum is prolonged backwards at the point where it is pierced by the siphuncle, forming a shelly tube somewhat like the neck of a bottle. In many fossil forms these septal necks are continued from the septum from which they arise to the next, so that the siphuncle is enclosed in a complete secondary calcareous tube. In the majority of Nautiloids the septal necks are directed backwards, and they are said to be retrosiphonate. In the majority of the Ammonoids the septal necks are continued forwards from the septa to which they belong, and such forms are termed prosiphonate.

The Tetrabranchiata were most abundant in the Palaeozoic and Mesozoic periods. The Nautiloidea are the most ancient, appearing first in the Upper Cambrian, the genera being most numerous in the Palaeozoic period, and comparatively few surviving into the Secondary. On the other hand, the Ammonoidea are scarce in Palaeozoic formations, being represented in deposits earlier than the Carboniferous only by comparatively simple types, such as _Clymenia_ and _Goniatites_. In the Secondary period Ammonoids were very abundant, both in genera and species and in individuals, and with few local exceptions none are known to have survived even to the commencement of the Tertiary. In the widest sense the genus _Nautilus_ has existed since the Ordovician (Silurian) period, but the oldest types are not properly to be placed in the same genus as the existing form. Even with this qualification the genus is very ancient, shells very similar to those of the living _Nautilus_ being found in the Upper Cretaceous.

It has been maintained by some zoologists that the Ammonoidea were Dibranchiate, though it would not follow from this that the shell was, therefore, internal. They are, however, generally classed with the Tetrabranchiata, and the absence of all evidence of the possession of an ink-sac is in favour of this view. There can be little doubt that they gave rise to the Dibranchiata.

About 2500 fossil species are included in the Nautiloidea, but only a few species of the genus _Nautilus_ survive. Some of the fossil forms are very large, the shell reaching a length of 2 metres, or 6 ft. 6 in. Of the Ammonoidea more than 5000 species have been described, and some of the coiled forms are 70 cm., or nearly 2 ft. 6 in. in diameter.

Associated with various forms of Ammonoids there have been found peculiar horny or calcified plates, sometimes contained within the body-chamber of the shell, sometimes wholly detached. The most typical form of these structures has been named _aptychus._ It consists of two bilaterally symmetrical halves, of somewhat semicircular shape, and attached to one another by their straight inner margins, like a pair of doors. In some cases the aptychus is thin and horny, but more often it is thick and calcified, in which case the principal layer has a peculiar cellular structure. The surface may be smooth or sculptured, and one side is usually marked by concentric lines of growth. Another type is similar, except that the two halves are united in the middle line; bodies of this character are called _synaptychus_; they occur in the body-chamber of species of _Scaphites_. Another form called _anaptychus_ consists of a thin horny undivided plate which is concentrically striated. This is associated with species of _Ammonites_ and _Goniatites_.

Many theories have been proposed in explanation of these structures. According to Sir Richard Owen, the aptychus is an operculum developed in a part of the body corresponding to the hood of _Nautilus_. E. Ray Lankester suggested that the double plate was borne on the surface of the nidamental gland, with the form and sculpturing of which in _Nautilus_ it closely agrees. On this view the aptychus would occur only in females. The most recent view is that these structures could not have been opercula because of their constant position inside the body-chamber, and that they were not external secretions at all, but a calcified internal cartilage situated at the base of the funnel.

_Classification of Tetrabranchiata._--Cephalopoda in which the mantle is entirely enclosed by a multilocular siphunculated shell, which may or may not be coiled. Only the last compartment of the shell occupied by the body of the animal. Numerous pedal tentacles around the mouth, which are retractile within sheaths. Halves of the funnel not united. Two pairs of ctenidia, and two pairs of renal tubes without reno-pericardial apertures. Pericardium opens directly to exterior. Cephalic cartilage wholly ventral. Optic vesicles with apertures, without crystalline lens.

_Sub-order 1. Nautiloidea_.--Initial chamber not inflated, with dorso-ventral cicatrix at extremity.

Fam. 1. _Orthoceratidae_. Shell straight or slightly curved, with a simple aperture, large terminal chamber and cylindrical siphuncle. _Orthoceras_, Silurian to Trias. _Baltoceras_, Silurian.

Fam. 2. _Actinoceratidae_. Shell straight or slightly curved, with wide siphuncle contracted at level of septa. _Actinoceras_, Silurian to Carboniferous. _Discosorus_, Silurian. _Huronia_, Silurian. _Loxoceras_, Silurian to Carboniferous.

Fam. 3. _Endoceratidae_. Shell straight, with wide margina siphuncle, necks produced into tubes fitting into one another. _Endoceras_, Silurian.

Fam. 4. _Gomphoceratidae_. Shell globular, straight or arcuate, aperture contracted. _Gomphoceras_, Silurian. _Phragmoceras_, Silurian.

Fam. 5. _Ascoceratidae_. Shell straight, ampulliform, summit truncate, terminal chamber extending nearly whole length of shell ventrally. _Ascoceras_, Silurian. _Glossoceras_, Silurian.

Fam. 6. _Poterioceratidae_. Shell straight or curved, fusiform, aperture simple, siphuncle contracted at septa. _Poterioceras_, Silurian to Carboniferous. _Streptoceras_, Silurian.

Fam. 7. _Cyrtoceratidae_. Shell slightly curved, aperture simple, siphuncle wide, septa approximated. _Cyrtoceras_, Devonian.

Fam. 8. _Lituitidae_. Shell coiled in one plane with the terminal part uncoiled, aperture contracted. _Lituites_, Silurian. _Ophidioceras_, Silurian.

Fam. 9. _Trochoceratidae_. Shell helicoidally coiled, dextral or sinistral, the last whorl generally uncoiled. _Trochoceras_, Devonian. _Adelphoceras_, Devonian.

Fam. 10. _Nautilidae_. Shell coiled in one plane, aperture wide and simple, siphuncle central. _Nautilus_, recent. _Trocholites_, Silurian. _Gyroceras_, Silurian to Carboniferous. _Hercoceras_, Silurian. _Ptenoceras_, Devonian. _Discites_, Carboniferous.

Fam. 11. _Bactritidae_. Shell straight, conical, siphuncle narrow and marginal, necks long, infundibuliform, sutures undulating. _Bactrites_, Silurian and Devonian.

_Sub-order 2. Ammonitoidea_,--Initial chamber spheroidal; siphuncle narrow and simple; septa convex towards aperture; sutures complex.

_Tribe 1. Retrosiphonata_.--Siphuncular necks projecting behind the septa as in Nautiloidea. Sutures form simple undulations. Occur exclusively in Palaeozoic strata from Devonian upwards.

Fam. 1. _Goniatitidae_. Shell nautiloid, with simple sutures and ventral siphuncle. _Goniatites_, Devonian and Carboniferous. _Anarcestes_, Devonian.

Fam. 2. _Clymeniidae_. Shell nautiloid, with simple sutures, siphuncle dorsal, that is, internal. _Clymenia_, Upper Devonian.

_Tribe 2. Prosiphonata._--Siphuncular necks projecting in front of the septa. Sutures form deeply indented lobes and saddles.

Fam. 1. _Arcestidae_. Globular and smooth or nearly smooth, with reduced umbilicus, terminal chamber very deep, an aptychus present. _Popanoceras_, Permian. _Cyclolobus_, Permian, _Arcestes_, Trias. _Lobites_, Trias.

Fam. 2. _Tropitidae_. Shells globular, but having radiating and tuberculated costae. _Thalassoceras_, Permian. _Tropites_, Trias. _Sibirites_, Trias.

Fam. 3. _Ceratitidae_. Shells coiled, with a large umbilicus, terminal chamber short, sutures with simple saddles. _Trachyceras_, Upper Trias. _Ceratites_, Trias. _Dinarites_, Trias.

Some genera with helicoidal shells are related to these coiled forms, viz. _Cochloceras_, Trias; also some straight forms, e.g. _Rhab-doccras_, Trias.

Fam. 4. _Pinacoceratidae_. Shell compressed, smooth, terminal chamber short, sutures very complicated, convex. _Pinacoceras_, Trias.

Fam. 5. _Phylloceratidae_. Shell coiled, the whorls overlapping each other, sutures formed of numerous lobes and saddles. _Phytloceras_, Jurassic.

Fam. 6. _Lytoceratidae_. Shell discoid, whorls loosely united or uncoiled, sutures deeply indented, but with only three saddles and lobes. _Lytoceras_, Jurassic and Cretaceous. _Macroscaphites_, Cretaceous. _Reunites_, Cretaceous. _Ptychoeeras_, Cretaceous. _Turrilites_, Cretaceous. _Baculites_, Cretaceous.

Fam. 7. _Ammonitidae_. Shell coiled, with narrow whorls which do not embrace one another, aperture simple, a horny anaptychus present. _Ammonites_, Jurassic. _Arietites_, Jurassic. _Aegoceras_, Lias.

Fam. 8. _Harpoceratidae_. Shell discord and flattened, with a carinated border, aperture provided with lateral projections, a calcareous aptychus, formed of two pieces. _Harpoceras_, Jurassic. _Oppelia_, Jurassic. _Lissoceras_, Jurassic and Cretaceous.

Fam. 9. _Amaltheidae_. Shell flattened, with a prominent carina continued anteriorly into a rostrum. _Amaltheus_, Lias. _Cardioceras_, Jurassic. _Schloenbachia_, Cretaceous.

Fam. 10. _Stephanoceratidae_. Shell not carinated, but with radiating costae, which are often bifurcated, aperture often with lateral projections which contract it, aptychus formed of two pieces. _Stephanoceras, Morphoceras, Pensphinctes, Peltoceras_, Jurassic. _Hoplites_, Cretaceous. _Acanthoceras_, Cretaceous. _Cosmoceras_, Jurassic. Various more or less uncoiled forms are related to this family, viz. _Scaphites, Crioceras_, Cretaceous.

ORDER 2. DIBRANCHIATA (= Holosiphona, Acetabulifera)

[Illustration: FIG. 15.--_Sepia officinalis_, L., about ½ natural size, as seen when dead, the long prehensile arms being withdrawn from the pouches at the side of the head, in which they are carried during life when not actually in use. a. Neck; b, lateral fin of the mantle-sac; c, the eight shorter arms of the fore-foot; d, the two long prehensile arms; e, the eyes.]

_Characters_.--Cephalopoda in which the inflected margins of the epipodia are fused so as to form a complete tubular siphon (fig. 24, i). The circumoral lobes of the fore-foot carry suckers disposed upon them in rows, _not_ tentacles (see figs. 15, 24). There is a single pair of typical ctenidia (fig. 25) acting as gills (hence Dibranchiata), and a single pair of renal organs, opening by apertures right and left of the median anus (fig. 25, r) and by similar internal pores into the pericardial chamber, which consequently does not open directly to the surface as in _Nautilus_. The oviducts are sometimes paired right and left (Octopoda, Oigopsida), sometimes that of one side only is developed (Myopsida). The sperm-duct is always single except, according to W. Keferstein, in _Eledone moschata._

A plate-like shell is developed in a closed sac formed by the mantle (figs 20, 21), except in the Octopoda, which have none, and in _Spirula_ (fig. 17, D) and the extinct _Belemnitidae_, &c., which have a small chambered shell resembling that of _Nautilus_ with or without the addition of plate-like and cylindrical accessory developments (fig. 17, A, C, fig. 19).

The pair of cephalic eyes are highly-developed vesicles with a refractive lens (fig. 33), cornea and lid-folds,--the vesicle being in the embryo, an open sac like that of _Nautilus_ (fig. 34). Osphradia are not present, but cephalic olfactory organs are recognized. One or two pairs of large salivary glands with long ducts are present. An ink-sac formed as a diverticulum of the rectum and opening near the anus is present in all Dibranchiata (fig. 25, t), and has been detected even in the fossil _Belemnitidae._ Branchial hearts are developed on the two branchial afferent blood-vessels (fig. 28, _vc'_, _vi_).

[Illustration: FIG. 16.--Decapodous Cephalopods.

A, _Cheiroteuthis Veranyi_, d'Orb. (from the Mediterranean).

B, _Thysanoteuthis rhombus_, Troschel (from Messina).

C, _Loligopsis cyclura_, Fér. and d'Orb. (from the Atlantic Ocean).]

[Illustration: FIG. 17.--Internal Shells of Cephalopoda.

A, _Conoteutliis dupiniana_, d'Orb. (from the Neocomian of France).

B, Shell _Sepia orbigniana_. Fér. (Mediterranean).

C, Shell of _Spirulirostra Bellardii_, d'Orb. (from the Miocene of Turin). The specimen is cut so as to show in section the chambered shell and the laminated "guard" deposited upon its surface.

D, Shell of _Splrula laevis_, Gray (New Zealand).]

In the Dibranchiata the shell shows various stages of degeneration, culminating in its complete disappearance in _Octopus_. As in other Mollusca, there is a tendency in Cephalopods for the mantle to extend over the outside of the shell from its edges, and when these secondary mantle-folds entirely cover the shell and meet or fuse together the shell is surrounded by the mantle both externally and internally, and is said to be internal, though it remains always a cuticular structure external to the epidermis. This procebs is generally accompanied by a reduction of the size of the shell in comparison with that of the body, so that the relations of the two are gradually reversed, the body outgrows its house and instead of the mantle being enclosed by the shell, the shell is enclosed by the mantle. The earliest stage of this process is shown in the recent _Spirula,_ though it is perhaps not impossible that in some of the later fossil Ammonoids the shell was becoming more and more internal. The shell of _Spirula_ (fig. 18) is coiled somewhat like that of _Nautilus_, but the coils are not in contact, the direction of the coil is endogastric or ventral instead of exogastric, and the shell is very much smaller than the body. Like that of _Nautilus_ it is divided by septa and traversed by a siphuncle. The relation of the animal to the terminal chamber is as in _Nautilus,_ but the body extends far beyond the aperture, and folds of the mantle grow up over the shell and cover it everywhere except part of the dorsal and ventral surfaces.

[Illustration: After Chun, from Lankester's _Treatise an Zoology_

FIG. 18.--_Spirula._

A, Dorsal aspect.

B, Ventral aspect.

a, Arms.

e, Eyes.

fi, Fins.

fu, Funnel.

pa, Mantle.

po, Posterior fossa.

sh, Shell.

te, Tentacular arms.

td, Terminal pallial disk]

[Illustration: FIG. 19.--Digram of shell Belemnite (after Phillips). r. Horny pen or "proostracum": A, conical cavity or "alveolus," in which the chambered "phragmacone" (p) is contained: g, "guard," or "rostrum."]

The next modification in the enclosed shell is the addition to it of secondary deposits of calcareous matter, by the inner surface of the shell-sac. Successive layers are deposited on the posterior part of the original shell, whether coiled or straight, and these layers form a conical mass, which may attain great thickness. A somewhat coiled shell with such a deposit is seen in _Spirulirostra_ (fig. 17, C) of the Miocene. In the next stage of modification secondary secretion forms a long and broad projection of the dorsal lip of the aperture; this is well developed in the belemnites (fig. 19). Thus in these modified shells three parts are to be distinguished: the original septate shell, which has been called the phragmacone; the posterior conical deposit, called the rostrum or guard; and the anterior somewhat flat projection, called the proostracum. In the living Dibranchiata other than _Spirula_ the phragmacone and rostrum have become very rudimentary. The shell of _Sepia_ (fig. 20) consists almost entirely of the proostracum, the little ventral hollow posteriorly representing the phragmacone, and the posterior pointed projection, the rostrum. In the _Oigopsida_ the shell is represented by a proostracum which is no longer calcified by forms a chitinous plume or gladuius, and a similar rudiment occurs in _Loliginidac_ (fig. 21) and _Sepiolidae_. Lastly, in the Octopoda the shell is represented only by small chitinous rudiments to which the retractor muscles of the head and funnel are attached; these are paired in _Octopus_, unpaired in other cases as in _Cirrhoteuthis_.

[Illustration: FIG. 20. FIG. 21. FIG. 20.--The calcareous internal shell of _Sepia officinalis, _the so-called cuttle-bone, a, Lateral expansion; b, anterior cancellated region; c, laminated region, the laminae enclosing air.]

[Illustration: FIG. 21.--The horny internal shell or gladius or pen of _Lohgo_.]

The early appearance of the sac of the mantle in which the shell is enclosed has led to an erroneous identification of this sac with the primitive shell-sac or shell-gland of the Molluscan embryo. The first appearance of the shell-sac in Dibranchiata is shown in figs. 35, 36. Its formation as an open upgrowth of the centro-dorsal area, and the fact that it appears and disappears without closing in _Argonauta_ and _Octopus_, was demonstrated by E. Ray Lankester.

[Illustration: FIG. 22.--The Argonaut in life. (After Lacaze-Duthiers) Tr. Float: Br.a, anterior arms: Br p, posterior arms: V, the expanded portion of them, once called the sails; B, the beak; C, the shell; En, the Funnel.]

In _Argonauta_ (the paper nautilus) the female only possesses a shell, in which the body is contained; but this is not homologous with the true shell in other cases; it is a structure _sui generis_ secreted by the expanded arms of the dorsal pair which are closely applied to it on either side (fig. 22).

[Illustration: FIG. 23.--Head and circumoral processes of the fore-foot of _Onychoteuthis_ (from Owen).

a, Neck.

b, Eye.

c, The eight short arms.

d, Long prehensile arms, the clavate extremities of which are provided with suckers at e, and with a double row of hooks beyond at f. The temporary conjunction of the arms by means of the suckers enables them to act in combination.]

[Illustration: FIG. 24.--Male of _Ocythoe catenulata_, Steenstrup (_Octopus carena_, Ver.), showing the hectocotylized arm. (From Gegenbaur.)

t^1, t^2, t^3, t^4, The first, second, third and fourth arms or processes of the fore-foot.

h, The third arm of the right side hectocotylized.

x, The apical sac of the hectocotylized arm.

y, The filament which issues from the sac when development is complete.

i, The siphon.]

_Head, Foot, Mantle and Mantle-cavity._--If we now compare the fore-foot of the Dibranchiata with that of _Nautilus_, we find in the first place a more simple arrangement of its lobes, which are either four or five pairs of tapering processes (called "arms"), arranged in a series around the buccal cone, and a substitution of suckers for tentacles on the surface of these lobes (figs. 15 and 24). The most dorsally placed pair of arms, corresponding to the two sides of the hood of _Nautilus_, are in reality the most anterior, and are termed the first pair. In the Octopoda there are four pairs of these arms (fig. 38), in the Decapoda five pairs, of which the fourth is greatly elongated (figs. 15, 16). In _Sepia_, _Sepiola_ and _Rossia_, each of these long arms is withdrawn into a pouch beside the head, and is only ejected for the purpose of prehension. In _Loligo_ they are completely retractile, very slightly so in the majority of the Oigopsida, and in _Rhynchoteuthis_ they are united to form a beak-like appendage. A gradual reduction of the tentacular arms can be seen in the Decapoda, leading to their total absence in Octopoda; thus in _Leachia_, _Chaunoteuthis_ and others these arms are reduced to mere stumps. In some _Cheiroteuthidae_ and _Cranchiidae_ the ordinary or sessile arms, especially the dorsal pairs, are reduced. In the Octopoda they are not unfrequently connected by a web, and form an efficient swimming-bell, e.g. in _Cirrhoteuthidae_ and _Amphuretidae_. The suckers are placed on the adoral surface of the arms, and may be in one, two or four rows, and very numerous. In place of suckers in some genera, e.g. _Veranya_, we find on certain arms or parts of the arms horny hooks; in other cases a hook rises from the centre of each sucker. The hooks on the long arms of _Onychoteuthis_ are drawn in fig. 23. In various species of _Cheiroteuthis_ the suckers on the tentacular arms are very feeble, but the bottom of the cup is covered by a number of anastomosed epithelial filaments which are used as a fishing-net. The fore-foot, with its apparatus of suckers and hooks, is in the Dibranchiata essentially a prehensile apparatus, though the whole series of arms in the Octopoda serve as swimming organs, and in many (e.g. the common octopus or poulp) the sucker-bearing surface is used as a crawling organ.

[Illustration: FIG. 25.--View of the postero-ventral surface of a male _Sepia_, obtained by cutting longitudinally the firm mantle-skirt and drawing the divided halves apart. This figure is strictly comparable with fig. 4. (From Gegenbaur.)

C, The head.

J, The mid-foot or siphon, which has been cut open so as to display the valve i.

R, The glandular tissue of the left nephridium or renal-sac, which has been cut open (see fig. 29).

P, P, The lateral fins of the mantle-skirt.

Br, The single pair of branchiae (ctenidia).

a, The anus--immediately below it is the opening of the ink-bag.

c, Cartilaginous socket in the siphon to receive c', the cartilaginous knob of the mantle-skirt--the two constituting the "pallial hinge apparatus" characteristic of Decapoda, not found in Octopoda.

g, The azygos genital papilla and aperture.

'i, Valve of the siphon (possibly the rudimentary hind-foot)

m, Muscular band connected with the fore-foot and mid-foot (siphon) and identical with the muscular mass k in fig. 3.

r, Renal papillae, carrying the apertures of the nephridia.

v.br, Branchial efferent blood-vessel.

v br', Bulbous enlargements of the branchial blood-vessels (see figs 28, 29).

t, Ink-bag]