732 



ZOOLOGY 



SECT. 



CO 



composed of pigmented and sensory cells. The latter (retinophores) 

 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 (retinulce) 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 (I.) of dense cuti- 

 cular matter secreted by the cells 

 of the retina ; this is surrounded 

 by a less dense vitreous body. 



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 continous with the; 

 groove traversing the penis. The oviduct (Fig. 613, 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 i 

 glandular walls, which passes forwards close to and parallel with 

 the rectum, and opens on the exterior near the anus. 



FIG. 618. Triton. Section of the eye- 

 co. cornea ; ep. epidermis ; 1. lens ; opt. 

 optic nerve ; rds. layer of rods (the line 

 is not continued far enough inwards) ; 

 ret. retina. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Gastropoda are unsymmetrical Mollusca, with a mantle 

 which is not divided into two lateral portions and usually 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 

 taking place through the wall of the mantle-cavity itself. A dis- 

 dinct head bearing eyes and tentacles is present in the majority. 

 The foot is situated behind the head, and usually has an extensive 

 flattened ventral surface. The buccal cavity contains an odoni 

 phore. The nephridium is usually single. The nervous systei 



