Part 17

In the males of the Dibranchiata one of the arms is more or less modified in connexion with the reproductive function, and is called the "hectocotylized arm." This name is derived from the condition assumed by the arm in those cases in which its modification is carried out to the greatest extent. These cases are those of the Octopods _Argonauta argo_ and _Ocythoe catenulata_ (fig. 24). In the males of these the third arm (on the left side in _Argonauta_, on the right side in _Ocythoe_) is found before the breeding season to be represented by a globular sac of integument. This sac bursts, and from it issues an arm larger than its neighbours, having a small sac at its extremity in _Ocythoe_ (fig. 24. x), from which subsequently a long filament issues. Before copulation the male charges this arm with the spermatophores or packets of spermatozoa removed from its generative orifice beneath the mantle-skirt, and during coitus the arm becomes detached and is left adhering to the female by means of its suckers. A new arm is formed at the cicatrix before the next breeding season. The female, being much larger than the male, swims away with the detached arm lodged beneath her mantle-skirt. There, in a way which is not understood, the fertilization of the eggs is effected. Specimens of the female _Ocythoë_ with the detached arm adherent were examined by Cuvier, who mistook the arm for a parasitic worm and gave to it the name _Hectocotylus_. Accordingly, the correspondingly modified arms of other Cephalopoda are said to be hectocotylized. J.J.S. Steenstrup has determined the hectocotylized condition of one or other of the arms in a number of male Dibranchs as follows:--in all, excepting _Argonauta_ and _Ocythoe_ and _Tremoctopus_, the modification of the arm is slight, consisting in a small enlargement of part or the whole of the arm, and the obliteration of some of its suckers; in _Octopus_ and _Eledone_ the third right arm is hectocotylized; in _Rossia_ and _Sepiola_ the fourth left arm is hectocotylized along its whole length, and the fourth right arm also in the middle only; in _Sepia_ the fourth left arm is modified at its base only; in _Sepioteuthis_, the same at its apex; in _Loligo_, the same also at its apex; in _Loliolus_, the same along its whole length; in _Ommatostrephes_, _Onychoteuthis_ and _Loligopsis_ no hectocotylized arm has hitherto been observed. Thus, speaking generally, it is one or both of the fourth pair of short arms which are modified in the Decapoda, of the third pair in the Octopoda. In the pallial cavity are situated one pair of gills in the Dibranchiata (fig. 25), attached dorsally along the whole of their afferent borders. On each side of the branchia is a series of lamellae, least in number in the Octopoda. Each lamella is transversely folded, and the folds are in turn folded, so that the respiratory surface is increased. On the somatic wall of the pallial cavity, between and ventral to the gills, are the following apertures: the anus and opening of the ink-sac, close together in the median line; a pair of apertures of the renal sacs, on either side of the median line; external to the renal orifice, on the left side, the genital aperture in _Cirrhoteuthidae_ and Myopsida. In other Octopoda, and in nearly all the Oigopsida among the Decapoda, the genital ducts are paired in the female, but only the left is developed in the male. The funnel forms a complete tube in the Dibranchiata, and in the majority of the Decapoda, as in _Nautilus_, it is provided with an internal valve projecting from its somatic surface, which allows water to pass outwards but prevents it passing inwards. The mantle performs rhythmical respiratory movements of expansion and contraction, the water entering between funnel and mantle and passing out through the funnel. In Decapoda the edge of the mantle bears internally on each side a cartilaginous projection which fits into a corresponding depression on the external surface of the funnel; this is called the "resisting apparatus," and serves to make the union of mantle and funnel firmer during expiration. More powerful expiratory movements are used for sudden retrograde locomotion through the water.

[Illustration: FIG. 26.--Diagram representing a vertical approximately median antero-posterior section of _Sepia officinalis_ (from a drawing by A.G. Bourne). The lettering corresponds with that of fig. 10, with which this drawing is intended to be compared.

a, Shell (here enclosed by a growth of the mantle).

b, The nuchal plate (here a cartilage).

c, (The reference line should be continued through the black area representing the shell to the outline below it), the integument covering the visceral hump.

d, The reflected portion of the mantle-skirt forming the sac which encloses the shell.

e, The inferior margin of the mantle-skirt (mouth of the pallial chamber).

f, The pallial chamber.

g, The vertically cut median portion of the siphon.

i, The valve of the siphon.

m, The two upper lobes of the fore-foot.

n, The long prehensile arms of the same.

o, The fifth or lowermost lobe of the fore-foot.

p, The third lobe of the fore-foot.

q, The buccal membrane.

v, The upper beak or jaw. s, The lower beak or jaw.

t, The lingual ribbon.

x, The viscero-pericardial sac.

n.c, The nerve-collar.

cr, The crop.

gizz, The gizzard.

an, The anus.

c.t, The left ctenidium or gill-plume.

vent, Ventricle of the heart.

a.b.v, Afferent branchial vessel.

e.b.v, Efferent branchial vessel.

re, Renal glandular mass.

n.n.a, Left nephridial aperture.

visc.per.apert, Viscero-pericardial aperture (see fig. 29).

br.b, Branchial heart.

app, Appendage of the same.

i.s, Ink-bag.]

_Luminous Organs._--In certain Oigopsida living in deep water, e.g. _Histioteuthis, Calliteuthis, Histiopsis, Pterygioteuthis_, the surface of the skin bears photogenous organs directed towards the oral extremity. Anatomically these consist of a deeper photogenous layer and a more superficial refracting layer. In some cases, e.g. _Pterygioteuthis_, they occur even within the mantle-cavity.

_Fins._--In the majority of the Decapoda and in the _Cirrhoteuthidae_, the mantle is produced into lateral symmetrical expansions which have the function of fins. They originate at the aboral extremity where they remain in _Spirula_ (fig. 18). In most other Oigopsida they are terminal, but more dorsal than ventral, e.g. _Loligopsis_ (fig. 16), and there may be two on each side, as in _Grimalditeuthis_. In other cases they extend laterally along a greater length of the body, as in _Sepia_ (fig. 15). In _Ctenopteryx_ they have a superficial resemblance to the fins of fishes, consisting of a thin membrane supported by a series of muscular rods.

_Chromatophores._--These are characteristic of the Dibranchiata, apparently absent in _Nautilus_. They are originally single cells of ectodermic origin which sink below the epidermis and become connected with radiating muscular fibres. The cells are single but multinuclear. Different cells contain pigments of different colours, yellow, brown, red or blue. Each cell in life is in constant tremulous movement; under the influence of nervous excitement the cells are suddenly expanded or contracted, producing blushes of colour and pallor. By reflex action of which the afferent stimulus acts upon the eyes as in fishes, the chromatophores assume a condition which approximates the colour of the animal to that of surrounding objects. In the Decapoda there are also reflecting elements which produce iridescent hues.

_Aquiferous Cavities._--In addition to the pockets into which the tentacular arms of Decapoda are retracted, there are in several Dibranchiata cavities in the integument which open to the exterior by special pores but have no communication with the vascular system or other internal cavities of the body. In _Ocythoe_ there are such pores on the back of the head and at the base of the funnel; buccal pouches on the ventral side of the mouth, internal to the arms, occur in some genera, one in _Loligo_, two in _Sepia_. In some species of _Sepia_ there are pouches in the mantle.

_Alimentary Tube._--The principal differences from _Nautilus_ are the following:--the mandibles are similar in shape, but are chitinous, not calcified. In the radula there are three teeth on each side of the median tooth in each row, except in _Gonatus_, in which there are only two lateral teeth, and the _Cirrhoteuthidae_, in which the radula has entirely disappeared. In front of the radula is the so-called tongue, a fleshy projection corresponding to the sub-radular organ of other Mollusca.

[Illustration: FIG. 27.--Alimentary canal of _Loligo sagittata_ (from Gegenbaur). The buccal mass is omitted.

oe, Oesophagus.

v, The stomach opened longitudinally.

x, Probe passed through the pylorus.

c, Commencement of the caecum.

e, Its spiral portion.

i, Intestine.

a, Ink-bag.

b, Its opening into the rectum.]

In most of the Dibranchiata there are two pairs of salivary glands. In the Decapoda the ducts of the posterior pair unite into a median duct which opens on the surface of the sub-radular organ. The anterior pair is but slightly developed except in the Oigopsida. In the Octopoda there are also two pairs, but the posterior pair, except in _Cirrhoteuthis_ where they are absent, are large and displaced backwards, being situated near the oesophageal proventriculus. Connected with the intestine immediately beyond the pylorus is a thin-walled caecum, spherical in _Rossia_ and _Leachia_, elongated in _Loligo_, but usually coiled into a spiral (fig. 27). The hepatic ducts open into the caecum. The liver is developed as a paired gland, more or less fused into one in the adult, but the ducts are always paired. The ducts are covered by a number of glandular follicles forming what is called the pancreas.

The ink-sac, absent in _Nautilus_, is a rectal caecum developed from its dorsal wall. It is present in all Dibranchiata except _Octopus arcticus, O. piscatorum_ and _Cirrhoteuthis_. It consists of a deeper part or gland proper and a reservoir. It extends to the posterior extremity of the body in _Sepia_, but in _Octopoda_ is usually embedded in the surface of the liver. The pigment of the secretion is melanin, and its function is to produce a dense opacity in the water, which conceals the animal.

_Vascular System_ (fig. 28).--The ventricle lies in the pericardial cavity, except in Octopoda where this cavity is much reduced. The auricles, one pair, are contractile expansions of the efferent branchial vessels. The heart gives off an anterior or cephalic and a posterior or abdominal aorta. The vascular system is almost perfect, arteries and veins being united by capillaries. The principal vein is a vena cava passing backwards ventrally from the cephalic region and dividing into two afferent branchial veins, each of which receives a pallial and an abdominal vein. Each of these afferent branchial vessels is enclosed in the cavity of a renal organ and is covered externally by the glandular tissue which forms the excretory part of the "kidney" (fig. 29). Each afferent vessel is expanded into a contractile branchial heart, which is provided with a glandular appendage. The latter corresponds to the glandular masses which are attached to the afferent branchial veins in _Nautilus_, and to the pericardial glands of other Molluscs.

[Illustration: FIG. 28.--Circulatory and excretory organs of _Sepia_ (from Gegenbaur, after John Hunter).

br, Branchiae (ctenidia).

c, Ventricle of the heart.

a, Anterior artery (aorta).

a', Posterior artery.

v, The right and left auricles (enlargements of the efferent branchial veins).

v', Efferent branchial vein on the free face of the gill-plume.

v.c, Vena cava.

vi, vc', Afferent branchial vessels (branches of the vena cava, see fig. 29).

vc", Abdominal veins.

x, Branchial hearts and appendages.

re, e, Glandular substance of the nephridia developed on the wall of the great veins on their way to the gills. The arrows indicate the direction of the blood-current.]

[Illustration: FIG. 29.--Diagram of the nephridial sacs, and the veins which run through them, in _Sepia officinalis_ (after Vigelius). The nephridial sacs are supposed to have their upper walls removed.

v.c, Vena cava.

r.d.v.c, Right descending branch of the same.

r.s.v.c, Left descending branch of the same.

v.b.a, Vein from the ink-bag.

v.m, Mesenteric vein.

v.g, Genital vein.

v.a.d, Right abdominal vein.

v.a.s, Left abdominal vein.

v.p.d, Right pallial vein.

v.p.s, Left pallial vein.

c.b, Branchial heart.

x, Appendage of the same.

c.v, Capsule of the branchial heart.

np, External aperture of the right nephridial sac.

y, Reno-pericardial orifice placing the left renal sac or nephridium in communication with the viscero-pericardial sac, the course of which below the nephridial sac is indicated by dotted lines.

y', The similar orifice of the right side.

a.r, Glandular renal outgrowths.

w.k, Viscero-pericardial sac (dotted outline).]

_Coelom._--The coelom forms a large sac with a constriction between the anterior or pericardial division and the posterior or genital division, and it is produced into lateral diverticula which contain the branchial hearts; but in the Octopoda the pericardial division is suppressed and the genital division communicates by long ducts with sacs containing the appendages of the branchial hearts. The renal sacs communicate with the pericardium by pores near the external renal apertures; in the Octopoda the reno-pericardial openings are in the capsules of the branchial hearts. The genital ducts pass from the genital coelom to the exterior. They are paired in female Oigopsida and Octopoda except _Cirrhoteuthidae_, but only the left persists in the males of all Dibranchiata, and in the female Myopsida.

[Illustration: FIGS. 30, 31.--Nerve-centres of _Octopus_. Figure 30 gives a view from the dorsal aspect, figure 31 one from the ventral aspect.

buc, The buccal mass.

ped, Pedal ganglion.

opt, Optic ganglion.

cer, Cerebral ganglion.

pl, Pleural ganglion.

visc, Visceral ganglion.

oes, Oesophagus.

f, Foramen in the nerve-mass formed by pedal, pleural and visceral ganglion-pairs, traversed by a blood-vessel.]

In the oviduct is a glandular enlargement, and in addition to this the females are provided with the so-called nidamental glands which are developed on the somatic wall of the pallial cavity, one on each side of the rectum, except in certain Oigopsida (_Enoploteuthis, Cranchia, Leachia_) and in the Octopoda, in which these organs are absent. The latter fact is related to the habit of the majority of the Octopoda of guarding or "incubating" their eggs, which have little protective covering. In the other cases the eggs are surrounded by a tough gelatinous elastic material secreted by the nidamental glands.

[Illustration: FIG. 32.--Lateral view of the nervous centres and nerves of the right side of _Octopus vulgaris_ (from a drawing by A.G. Bourne).

bg, Buccal ganglion.

cer, Cerebral ganglion.

ped, Pedal ganglion.

pl, Pleural, and visc., visceral region of the pleuro-visceral ganglion.

gang. stell, The right stellate ganglion of the mantle connected by a nerve to the pleural portion.

n.visc, The right visceral nerve.

n.olf, Its (probably) olfactory branches.

n.br, Its branchial branches.]

The vas deferens is at first narrow and convoluted, then dilates into a vesicula seminalis at the end of which is a glandular diverticulum called the prostate. By the vesicula and the prostate the spermatophores are formed. These have a structure similar to those of _Nautilus_, and in the Octopoda may be as much as 50 mm. in length. Beyond the prostate the duct opens into a large terminal reservoir which has been called Needham's sac, and in which the spermatophores are stored.

_Nervous System and Sense-Organs._--The figures (30, 31, 32) representing the nerve-centres of _Octopus_ serve to exhibit the disposition of these parts in the Dibranchiata. The ganglia are more distinctly swollen than in _Nautilus_. In _Octopus_ an infra-buccal ganglion-pair are present, corresponding to the buccal ganglion-pair of Gastropoda. In Decapoda a supra-buccal ganglion-pair connected with these are also developed. Instead of the numerous radiating pallial nerves of _Nautilus_, we have in the Dibranchiata on each side (right and left) a large pleural nerve passing from the pleural portion of the pleuro-visceral ganglion to the mantle, where it enlarges to form the stellate ganglion. From each stellate ganglion nerves radiate to supply the powerful muscles of the mantle-skirt. The two stellate ganglia are connected, except in _Sepiola_, by a transverse supra-oesophageal commissure, which represents the pallial cords united by a commissure above the intestine in Amphineura. The nerves from the visceral portion of the pleuro-visceral ganglion have the same course as in _Nautilus_, but no osphradial papilla is present. An enteric nervous system is richly developed in the Dibranchiata, connected with the somatic nervous centres through the buccal ganglia, as in the Arthropoda through the stomato-gastric ganglia, and anastomozing with deep branches of the visceral nerves of the viscero-pleural ganglion-pair. It has been especially described by A. Hancock in _Ommatostrephes_. Upon the stomach it forms a single large and readily detected gastric ganglion.

[Illustration: FIG. 33.--Horizontal section of the eye of _Sepia_ (Myopsid). (From Gegenbaur, after Hensen.)

KK, Cephalic cartilages (see fig. 8).

C, Cornea (closed).

L, Lens.

ci, Ciliary body.

Ri, Internal layer of the retina.

Re, External layer of the retina.

p, Pigment between these.

o, Optic nerve.

go, Optic ganglion.

k and k', Capsular cartilage.

ik, Cartilage of the iris.

w, White body.

ae, Argentine integument.]

In the Dibranchiate division of the Cephalopoda the greatest elaboration of the dioptric apparatus of the eye is attained, so that we have in this class the extremes of the two lines of development of the Molluscan eye, those two lines being the punctigerous and the lentigerous. The structure of the Dibranchiate's eye is shown in section in fig. 14, C, and in fig. 33, and its development in figs. 34 and 37. The open sac which forms the retina of the young Dibranchiate closes up, and constitutes the posterior chamber of the eye, or primitive optic vesicle (fig. 37, A, poc). The lens forms as a structureless growth, secreted by both the internal and external surfaces of the front wall of the optic vesicle (fig. 37, B, l). The integument around the primitive optic vesicle which has sunk below the surface now rises up and forms firstly nearest the axis of the eye the iridian folds (if in B, fig. 37; ik in fig. 33; Ir in fig. 14), and then secondly an outer circular fold grows up like a wall and completely closes over the iridian folds and the axis of the primitive vesicle (fig. 33, C). This covering is transparent, and is the cornea. In the oceanic Decapoda the cornea does not completely close, but leaves a central aperture traversed by the optic axis. These forms are termed Oigopsidae by C. d'Orbigny, whilst the Decapoda with closed cornea are termed Myopsidae. In the Octopoda the cornea is closed, and there is yet another fold thrown over the eye. The skin surrounding the cornea presents a free circular margin, and can be drawn over the surface of the cornea by a sphincter muscle. It thus acts as an adjustable diaphragm, exactly similar in movement to the iris of Vertebrates. _Sepia_ and allied Decapods have a horizontal lower eyelid, that is to say, only one-half of the sphincter-like fold of integument is movable. The statocysts are situated ventrally between the pedal and visceral ganglia, and are entirely enclosed in the cranial cartilage. The cavity of each is continued into a small blind process which is the remnant of the embryonic connexion of the vesicle with the external surface. The sensory epithelium is at the anterior end of the vesicle forming a macula acustica, and in the cavity is a single otolith, partly calcareous and partly organic except in _Eledone_, in which it is entirely organic. The nerve arises from the cerebral ganglion on each side and passes through the pedal ganglion.

There is no branchial osphradium in the Dibranchiata corresponding to that of _Nautilus_, but the olfactory organ or rhinophore near the eye is present. In _Sepia_ and the majority of the Dibranchiata it is a simple pit, in some of the Oigopsida it is a projection which may be stalked.

[Illustration: FIG. 34.--Diagrams of sections showing the early stage of development of the eye of _Loligo_ when it is, like the permanent eye of _Nautilus_ and of _Patella_, an open sac. (From Lankester.)

A, First appearance of the eye as a ring-like upgrowth.

B, Ingrowth of the ring-like wall so as to form a sac, the primitive optic vesicle of _Loligo_.]

_Reproduction and Development._--The modification of one or a pair of the arms in the male for purposes of copulation has already been described. In many genera the sexes differ from one another in other characters also. As a rule the males are more slender or smaller than the females. The maximum degree of sexual dimorphism occurs in _Argonauta_ among the Octopods; in this genus the female may be fifteen times as large as the male, and the peculiar modification of the dorsal arms for the secretion of the shell occurs in the female only, no shell being formed in the male. In most cases the females are much more numerous than the males, but the opposite relation appears to exist in those Octopoda in which the hectocotylus is autotomous, for as many as four hectocotyli have been found in the pallial cavity of a single female. When the hectocotylus is not detached it is usually inserted into the pallial cavity of the female so as to deposit the spermatophores in or near the aperture of the oviduct, but in _Sepia_ and _Loligo_ they are merely deposited on the ventral lobes of the buccal membrane.

The eggs are laid shortly after copulation. In the Octopoda and in _Sepia_, _Sepiola_ and _Rossia_, each egg has a separate envelope continued into a long stalk by which it is attached with several others in a cluster. In _Argonauta_ the eggs are carried by the female in the cavity of the shell. In _Loligo_ the eggs are very numerous, and are enclosed in cylindrical transparent gelatinous strings united at one end into a cluster.

The Cephalopoda appear to be the only Invertebrates in which the egg is mesoblastic and telolecithal like that of Vertebrata. This is the result of the large quantity of the yolk, and the position the latter assumes in relation to the blastoderm. In all other Mollusca the segmentation is complete though in some cases very unequal. In the egg of _Loligo_, which has been chiefly studied (fig. 35), the protoplasmic pole is at the narrower end of the egg, and segmentation is restricted to this end, forming a layer of ectoderm cells. From one part of the periphery of the ectoderm proliferation of cells takes place and gives rise to a layer of scattered nuclei over the whole surface of the yolk. The region of proliferation marks the anal side of the ectoderm, and the layer of nuclei forms the perivitelline membrane. This process must be regarded as equivalent to the first stage of invagination, the yolk being surrounded by hypoblast cells or their nuclei. Later on the same anal edge of the ectoderm forms another cellular layer, the endoderm proper, which forms a continuous sheet below the ectoderm.

The mesoderm also originates at the anal side of the ectoderm and extends in two bands right and left between ectoderm and endoderm. After the mesoderm is thus established, a little vesicle lying upon and open to the yolk is formed from the endoderm, and this vesicle ultimately gives rise to the stomach, the two lobes of the liver and the intestine. The buccal mass and oesophagus arise from a stomodaeal invagination, and the anus is formed later from a short proctodaeal invagination.