682 



ZOOLOGY 



SK<T. 



CO 



rds 



The organs of special sense of Triton, in addition to the 

 tentacles and the osphradium, which have been already referred 

 to, are the eyes and the otocysts. The eye (Fig. 581) is a rounded 

 invagination of the epidermis with an inner wall or retina (ret.} 



composed of pigmented and sen- 

 sory cells. The latter (retinophorcs) 

 are elongated cells narrowed at 

 their central free ends, and pro- 

 duced at the opposite extremity 

 to become continuous with nerve- 

 fibres of the optic nerve. The 

 former (rctimihc) have their free 

 extremities much enlarged, and 

 surround the slender ends of the 

 retinophores. A layer of short 

 rods (rds.) lies within the retina 

 proper. The outer wall is thin, 

 and, with the overlying epidermis, 

 forms a transparent cornea. In 

 the interior of the eye is a clear 

 rounded lens (/.) of dense cuticular 

 matter secreted by the cells of 

 the retina ; this is surrounded 

 by a less dense body the vitreous. 



The sexes are distinct. There is- a single gonad ovary or tcstis 

 as the case may be lodged in the visceral spiral. The sperm-duct 

 is a white tube, thickish and much convoluted where it leaves the 

 testis, narrower and straight distally ; it opens in front in the 

 mantle-cavity into the proximal end of the sperm -groove, which, as 

 already mentioned, runs forwards along the right side, and becomes 

 continuous with the groove traversing the penis. The oviduct 

 (Fig. 576, ovid.) is proximally a very delicate tube with colourless 

 transparent walls. This runs forwards to the right side of the 

 mantle-cavity, where it assumes the character of a stout tube 

 (ovid.') with thickened glandular walls, which passes forwards close 

 to and parallel with the rectum, and opens on the exterior near 

 the anus. 



ret 



FIG. 581. Triton. Section of the eye. 

 co. cornea; ep. epidermis ; I. lens ; <>/>/. 

 optic nerve ; >\ls. layers of rods (the line 

 is not continued far enough inwards) ; 

 rut. retina. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Gastropoda are unsymmetrical Mollusca, with a mantle 

 which is not divided into two lateral portions, and with a shell 

 which does not consist of two lateral valves, but of a single, un- 

 symmetrical, usually spirally coiled, valve, enclosing a visceral mass 

 of corresponding form. There are, typically, two plume-like 

 ctenidia enclosed in a mantle-cavity, but there may be only one ; 

 and in air-breathing forms ctenidia are not developed, respiration 



