xi PHYLUM MOLLUSCA 295 



otocysts, osphradia or water-testing organs, and usually 

 olfactory organs in the shape of special groups of cells on 

 the tentacles. 



The nephridia are glandular tubes or chambers com- 

 municating, as in the Fresh-water Mussel, with the peri- 

 cardial cavity on the one hand, and with the exterior on 

 the other. Two nephridia, right and left, may be present, 

 or only one. 



The sexes are separate in some Gastropoda ; in others, 

 such as the Snails and Slugs, they are united ; and in the 

 latter case the structure of the reproductive organs is highly 

 complex. The larva is a Trochosphere, which subsequently 

 develops into a form known as the Veliger. In the Veliger 

 the prototroch, or ciliated prae-oral ridge of the Trocho- 

 sphere, becomes drawn out into a bilobed flap bordered 

 with strong cilia. There is a shell, a distinct foot bearing 

 an operculum, and tentacles and eyes are present on the 

 head region. The shell is at first of simple conical form, 

 and the anus is placed in the middle line posteriorly. It 

 is only as development advances that one side of the body 

 becomes more rapidly developed than the other, and the 

 anus becomes shifted forwards, the shell at the same time 

 in the great majority taking on a spiral form, and the 

 visceral prominence enclosed within it acquiring a corre- 

 sponding shape. 



Two main divisions or sub-classes of the Gastropoda are 

 recognised the Streptoneura and the Euthyneura. The 

 former comprises the majority of the marine Univalves, 

 such as the Limpets, Ear-shells, Cowries, Tritons, Whelks 

 and Cones. The latter includes the water-breathing Sea- 

 hares and Nudibranchs and the air-breathing Snails and 

 Slugs. The chief general points of distinction between the 

 two groups are that in the Streptoneura the visceral nerve 



