144 



MOLLUSCA 



lens forms as a structureless growth, projecting inwajds from 

 the front wall of tMs vesicle (fig. 123, B, I). The integument 

 around the primitive optic vesicle which has sunk below 



A 



PiQ 119 Diagrams of sections showing the eai-ly stage of development of the 



eye of Loligo when it is, like the permanent eye of Nautilus and of Patella, 

 an open sao. A. First appearance of tlie eye as a ring-like upgrowth. B. 

 Ingrowth of the ring-like wall so as to form a sao, the primitive optic vesicle 

 of liOligo. (From Lankester.) 



the surface now rises up and forms firstly nearest the axis 

 of the eye the iridian folds (if in B, fig. 123 ; ikva fig. 120 ; 

 Jr in fig. 118), 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. 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 Oigopsidge 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 



7% 



Fig 120 —Horizontal section of the eye of Sepia (Myopsid). EK, cephahc 

 caxtila'ges (see flg. 116); C, cornea (closed); i, lens; ci, ciliary body; Si, 

 internal layer of the retina ; Be, external layer of the "retma ; p, pigment 

 between these ; o, optic nerve ; go, optic ganglion ; fcandJ/, capsular cartilage ; 

 ik, cartilage of the iris ; w, white body ; ae, argentine integument. (From 

 Gegenbaur, after Hensen.) 



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. 120, 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 " ciliary body," 

 in which muscular tissue is present, serving to regulate the 

 focus of the lens (d 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 (^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 sis 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 brilKance. 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. AU 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 Duets. — ^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 " pyriform appendage," 

 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-MoUusc 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 fiUs. 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 



