The Cambridge natural history . xtremely simple to theelaborately complex. It may l)e, as i]icertain l)ivalves, no more than a, pig-mented spot on the mantle, or it mayconsist, as in some of the Cephalopoda,of a Cornea, a sclerotic, a choroid, aniris, a lens, an acpieous and vitreousluunour, a retina, and an optic nerve, orof some of these parts only. In most land and fresh-water Mol- lusca the eye may be regarded, roughlyspeaking, as a ball connected l)y anexceedingly fine thread (the optic nerve) ^^^ ^~with a nerve centre (the cerebral gang-lion). In PcducUna this ball is elliptic,in PlanorM
The Cambridge natural history . xtremely simple to theelaborately complex. It may l)e, as i]icertain l)ivalves, no more than a, pig-mented spot on the mantle, or it mayconsist, as in some of the Cephalopoda,of a Cornea, a sclerotic, a choroid, aniris, a lens, an acpieous and vitreousluunour, a retina, and an optic nerve, orof some of these parts only. In most land and fresh-water Mol- lusca the eye may be regarded, roughlyspeaking, as a ball connected l)y anexceedingly fine thread (the optic nerve) ^^^ ^~with a nerve centre (the cerebral gang-lion). In PcducUna this ball is elliptic,in PlanorMs and Neritina it is drawnout at the back into a conical or pearshape. In Helix (Fig. 87) there is a. structureless meml)rane,.siuTounding tlie whole eye, a lens, and a retina, the latterconsisting of a nervous layer, a cellular layer, and a layer ofrods containing pigment, tliis innermost layer (that nearest thelens) being of the thickness of lialf the whole retina. Comparing the eyes of different Gasteropoda together, we find. —Eye of Helix pmnatiaL., retracted within thetentacle ; c. cornea ; c}), epi-thelial layer; I, lens; ,optic nerve ; /•. retina. (AfterSimroth.) I82 STAGES OF DEVELOPMENT IX THE EYE CHAP. that they represent stages in a general course of in Patella the eye is scarcely more than an invaginationor depression in the integument, which is lined with pig-mented and retinal cells. The next upward stage occurs inTrocJiUS, where the depression becomes deeper and hladder-shaped, and is filled with a gelatinous or crystalline mass, l:)utstill is open at the top, and therefore permits tlie eye to he
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
