EMBRYONIC DEVELOPMENT OF GASTEROPODS. 



611 



entire history of the development of the fresh- water Mussel as 

 successfully as M. Lacaze Duthiers has worked out an important 

 part of that of the common Mytilus edulis or true Mussel.* 



Fig. 322. 



Parasitic Larva 'Glochidium) of Anodon: — A, Glochidia attached 

 to the tail of a Stickleback ; b, side view of Glochidium still en- 

 closed in the egg-rnenibrane, showing the hooks of its valves and 

 the byssus-filament a ; c, Glochidium with its valves widely 

 opened, showing the adductor-muscle a ; t>, side view of Glochi- 

 dium, with the valves opened to show the origin of the byssus- 

 filament and the three pairs of tentacular ..'?' organs, the barbed 

 hooks b, and the muscular or membranous folds c, c, connected 

 with them. 



476. The history of Embryonic Development may he studied with 

 peculiar facility in certain members of the Class of Gasteropods, 

 and presents numerous phenomena of great interest. The Eggs 

 (save among the Terrestrial species) are usually deposited in aggre- 

 gate masses, each enclosed in a common protective envelope or 

 Nidamentum. The nature of this Envelope, however, varies 



* See his admirable ' Memoire sur le Developpement des Branchies des 

 Mollusques Acephales Lamellibranches,' in "Ann. des Sciences Nat.," 

 Ser. 4, Tom. v. (1856), p. 5. 



R R 2 



