6 MOLLUSCA, 



and arranged also in a spiral manner. In Doris they are 

 in a spiral band. In Tergipes the egg-mass is saccate ; 

 in Eolis, funiculate. 



In the air-breathing Gasteropods also the nidus varies. 

 In the Limnaida. it is transparent, gelatinous, and lar- 

 viform, and the eggs are arranged in two layers, one 

 above the other. In Fhysa it is oval and smooth. In 

 Coretus it is shield-shaped, striated, and unattached. 

 In Ancylus it is circular, flat beneath, and arched above, 

 with the three-cornered eggs arranged in a circle in the 

 middle. In BuUnus it is vermiform, free, and transparent. 



In the terrestrial Gasteropods the eggs are usually 

 spherical, opaque, and extruded separately. In Onchidium 

 occidentale they are oblong, connected by a filament, 

 rolled together in a ball, and deposited in shady places. 

 In Limax rufus they are isolated, oblong, opaque, with 

 white coriaceous shells, and heaped together under stones 

 in moist places. In Succinea ampJiibia they are circular 

 and transparent, without nidus, but united in a mass. 

 In Helix pomatia they are opaque, white, and leathery, 

 varying in number from thirty to thirty-six, and are 

 deposited in holes in the ground ; while in the tropical 

 Bidimi they are often very large, oval, and calcareous, 

 and sometimes protected by an artificial nest formed of 

 leaves. The colour of the eggs of the air-breathing 

 Gasteropods is usually bluish or milk-white ; those, how- 

 ever, of Achatina, Helix hicarinata, and H. purpurea are 

 of a beautiful yellow. They also vary greatly in size. 

 In the common Snail they are no bigger than a mustard- 

 seed, while in Bulimiis hcemastonius they are almost as 

 large as a pigeon''s egg, with a hard calcareous shell. 



The discovery of the metamorphosis of molluscous 

 animals was first made by M. Sars in the Nudibranchs, 



