82 COMPARATIVE ANATOMY 



spermatozoa are lodged. In copulation the spermophore of 

 one snail is transferred to the spermatheca of the other, where 

 after a while it is dissolved and the spermatozoa are stored in 

 the terminal dilatation. As the ova pass down the vagina they 

 are fertilised by spermatozoa injected from the spermatheca, 

 and thus cross-fertilisation is effected. 



Snails deposit their eggs in the ground in masses during the 

 summer months, and so effectually conceal the holes in which 

 they have laid them that it is almost impossible to discover 

 them. After the ground has been softened by a shower of 

 rain, however, it is not uncommon to find a snail partially 

 buried in the ground. It is then engaged in excavating a hole 

 in which to lay its eggs, and if disturbed will quit the spot 

 without effecting its purpose. But the place may be marked 

 without disturbing the animal, and on the following day the 

 eggs may be found some three or four inches below the 

 surface. They are about the size of peas, with hard calcareous 

 shells, and as many as sixty or eighty may be deposited in a 

 single hole. Though the egg is large the ovum itself is minute, 

 the greater part of the contents of the egg-shell consisting of 

 albumen, which serves as food for the embryo during develop- 

 ment. The most important phases of development take place 

 within the egg-shell, and the young snail is not hatched till it 

 has assumed the characters of the adult. 



Though the ova of Helix and its allies, the slugs, are fairly 

 convenient objects for study, the details of their development 

 can hardly be understood without reference to the larval forms 

 characteristic both of marine lamellibranchs and gastropods. 

 In these animals the young quit the egg at a very early 

 stage in the form of a free swimming ciliated larva whose 

 organisation is so similar to that of the trochosphere 

 larvae of Polygordius and polychaete worms that we cannot 

 regard the resemblances as accidental, but as evidence 

 that the Annelids and Molluscs have descended from a 

 common ancestor. We may therefore omit the development 

 of the snail in order to study the more characteristic and 

 important life-history of a marine gastropod. The details of 

 development, however, differ so considerably in different 

 species that it will serve our present purpose best if we try to 

 gain a general idea of gastropod development without confining 

 our attention to any particular species. 



