Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/709

Rh CEPHALOPODA.] MOLLUSCA Gbl lens forms as a structureless growth, projecting inwards from the front wall of this vesicle (fig. 123, B, I). The integument around the primitive optic vesicle which has sunk below A Fir,. 119. 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. 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. (From Lankester.) the surface now rises up and forms firstly nearest the axis of the eye the iridian folds (if in B, fig. 123 ; ik in fig. 120 ; IT in fig. 118), and then secondly an outer circular fold grows up like a Avail and completely closes over the iridian folds and the axis of the primitive vesicle (fig. 120, 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 d Orbigny (42), whilst the Decapoda with closed cornea are termed Myopsidse. 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 fi Fio. 120. Horizontal section of the eye of Sepia (Myopsid). KK, cephalic cartilages (see fig. 116) ; 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 ; fcand k, capsular cartilage ; ik, cartilage of the iris ; w, white body ; ae, argentine integument. (From Uegenbaur, after Heusen.) movement to the iris of Vertebrates. Sepia and allied Deca pods have a horizontal lower eyelid, that is to say, only one-half of the sphincter-like fold of integument is movable. The exact history of the later growth of the lens in the Dibranchs eye is not clear. As seen in fig. 1 20, it appears, after attaining a certain size, to push through the front wall of the primitive optic vesicle at the point correspond ing to its centre of closure, and to project a little into the anterior chamber formed by the cornea. The wall of the primitive optic vesicle adjacent to the embedded lens (L) now becomes modified, forming a so-called &quot;ciliary body,&quot; in which muscular tissue is present, serving to regulate the focus of the lens (ci in fig. 120). Bobretzky (43) differs from Lankester, whose view is above given, in assigning a distinct origin to the protruding anterior segment of the lens (I 1 in fig. 118). The optic ganglion, as well as the other large ganglia of the Dibranchiata, originate in the mesoblast of the embryo. The connexion between the cells of the retina and the nerve- fibres proceeding from the optic ganglion must therefore be a secondary one. Chromatophores. In Nautilus these remarkable struc tures, which we mention here as being intimately asso ciated with the nervous system, appear to be absent. In Dibranchiata they play an important part in the economy, enabling their possessor, in conjunction with the discharge of the contents of the ink-bag, to elude the observation of either prey or foe. They consist of large vesicular cells (true nucleated cells converted into vesicles), arranged in a layer immediately below the epidermis. Each chroma- tophore-cell has from six to ten muscular bands attached to its walls, radiating from it star-wise. The contraction of these fibres causes the chromatophore-cell to widen out ; it returns to its spherical resting state by its own elasticity. In the spherical resting state such a cell may measure 01 mm., whilst when fully stretched by its radiat ing muscles it covers an area of 5 mm. The substance of the chromatophore-cells is intensely coloured with one of the following colours scarlet, yellow, blue, brown which are usually of the greatest purity and brilliance. The action of the chromatophores may be watched most readily in young Loligo, either under the microscope or with the naked eye. The chromatophores are suddenly expanded, and more slowly retracted with rapidly-recurring alter nation. All the blue, or all the red, or all the yellow may be expanded and the other colours left quiescent. Thus the animal can assume any particular hue, and change its appearance in a dazzling way with extraordinary rapidity. There is a definite adaptation of the colour assumed in the case of Sepia and others to the colour of the surrounding rock and bottom. Gonads and Genital Ducts. In Nautilus it has recently been shown by Lankester and Bourne (37) that the genital ducts of both sexes are paired right and left, the left duct being rudimentary and forming the &quot;pyriform appendage,&quot; described by Owen as adhering by membranous attach ment to the ventricle of the heart, and shown by Kefer- stein to communicate by a pore with the exterior. Thus the Cephalopoda agree with our archi-Mollusc in having bilaterally symmetrical genital ducts in the case of the most archaic member of the class. The ovary (female gonad) or the testis (male gonad) lies in Nautilus as in the Dibranchs in a distinct cavity walled off from the other viscera, near the centro-dorsal region. This chamber is formed by the coelomic or peritoneal wall ; the space enclosed is originally part of the coelom, and in Sepia and Loligo is, in the adult, part of the viscero-pericardial chamber. In Octopus it is this genital chamber which communicates by a right and a left canal with the nephrid- ium, and is the only representative of pericardium. The ovary or testis is itself a growth from the inner wall of this chamber, which it only partly fills. In Nautilus the right genital duct, which is functional, is a simple continuation to the pore on the postero-dorsal surface of the membran ous walls of the capsule in which lies the ovary or the testis, as the case may be. The gonad itself appears to represent a single median or bilateral organ. The true morphological nature of the genital ducts of the Cephalopoda and of other Mollusca is a subject which invites speculation and inquiry. In all the cases in which such YVT