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.

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.

Fig. 35.—Development of Loligo.

1. View of the cleavage of the egg during the first formationof embryonic cells.

2. Lateral view of the egg at a little later stage. a, Limitto which the layer of cleavage-cells has spread over the egg;b, portion of the egg (shaded) as yet uncovered by cleavage-cells;ap, the auto-plasts; kp, cleavage-pole where first cells were formed.

3. Later stage, the limit (a) now extended so as to leave butlittle of the egg-surface (b) unenclosed. The eyes (d),mouth (e) and mantle-sac (u) have appeared.

4. Later stage, anterior surface, the embryo is becomingnipped off from the yolk-sac (g).

5. View of an embryo similar to (3) from the cleavage-poleor centro-dorsal area.

6. Later stage, posterior surface.

7. Section in a median dorso-ventral and antero-posteriorplane of an embryo of the same age as (4).

8. View of the anterior face of an older embryo.

9. View of the posterior face of an embryo of the same age as (8).

Letters in (3) to (9):—a, lateral fins of the mantle;b, mantle-skirt;c, supra-ocular invagination to form the “white body”;d, the eye; e, the mouth; f1, f2, f3, f4, f5, thefive paired processes of the fore-foot; g, rhythmicallycontractile area of the yolk-sac, which is itself a hernia-likeprotrusion of the median portion of the fore-foot;h, dotted line showing internal area occupied by yolk(food-material of the egg); k, first rudiment of the epipodia(paired ridges which unite to form the siphon or funnel); l, sacof the radula or lingual ribbon; m, stomach; n, rudiments of thegills (paired ctenidia); o, the otocysts—a pair of invaginationsof the surface of the epipodia; p, the optic ganglion;q, the distal portion of the ridges which form the siphon,k being the basal portion of the same structure; r, the vesicle-likerudiment of the intestineformed independently of theparts connected with themouth, s, k, m, and withoutinvagination; s, rudimentof the salivary glands; t in(7), the shell-sac at an earlierstage open (see fig. 36), nowclosed up; u, the open shell-sacformed by an uprisingring-like growth of the centro-dorsalarea; w in (5), themantle-skirt commencing tobe raised up around the areaof the shell-sac. In (7) mespoints to the middle cell-layerof the embryo, ep tothe outer layer, and h to thedeep layer of fusiform cellswhich separates everywherethe embryo from the yolk orfood-material lying within it.

The external changes of form are as follows:—The mantle is the middle of the embryonic area, and in its centre is the shell-gland, which, however, behaves in a different way from that seen in other Molluscs. Its borders grow inwards and approach each other to form the shell-sac. E. Ray Lankester showed that in Argonauta and other Octopods the shell-sac disappears before it is closed up, but in other forms except Spirula it closes completely and the shell develops within it. The lateral and posterior borders of the embryo form the foot, and these borders grow out into ten or eight lobes which become the arms, and which at first, as seen in fig. 35 (8), are entirely posterior to the mouth. Development actually shows the anterior arms gradually growing round the mouth and uniting in front of it. Between the mantle and the foot are two ridges which form the funnel, and their position shows them to be the epipodia. The otocysts and eyes are formed as invaginations of ectoderm, the former behind the eyes, at the sides of the funnel. All the nerve-centres, cerebral, visceral, pedal and optic, are formed as proliferations of the ectoderm. At the sides of the optic ganglia a pair of ectodermic invaginations are formed, which in the adult become the white bodies of the eyes, surrounding the optic ganglion. These are vestiges of lateral cerebral lobes which degenerate in the course of development.

The coelomic cavity appears as a symmetrical pair of spaces in the mesoderm, right and left of the intestine, and from it grow out the genital ducts and the renal organs. The gonad develops from the wall of the coelom.

Fig. 36.—Section through aboral end of embryo of Loligo showing shell-sac still open. ep, ectoderm; m, mesoderm; m′, endoderm; shs, shell-sac; y, yolk.