MOLLl S< \ 



237 



foot is capable of swelling to almost twice that diameter. It 

 feeds largely on clams, boring a hole in the shell very much as 

 Urosalpinx attacks the oyster. Certain tropical snails, notably 

 the genus Conns, are said to have a poisonous bite, but probably 

 not fatal except to small animals. Another group of tropical 

 Gastropoda is interesting 



because the shells, con- 

 sisting of layers of differ- 

 ent colors, are used in the 

 manufacture of cameos. 

 They belong to the family 

 Cassidae, called commonly 

 helmet shells ; the one 

 most used is Cassis mada- 





i^dscarensts. 



V 



Finally, we include in 

 this order the curious 

 transparent M o 1 1 u s c a 

 called the Heteropoda, 

 common in tropical waters 

 (Fig. 243). The body is 

 more or less cylindrical 

 and the foot is compressed 

 laterally into a sort of fin, 

 by means of which the 

 animal swims. On the 

 dorsal side of the body 

 the viscera may be found 

 in a small hump inclosed 

 in a simple shell, or the 

 shell may be entirely ab- 

 sent. They are jellylike 

 animals, pelagic, and often 

 attain a length of from fifteen to twenty centimeters 



fc 



1 



FlG.242. Fulgur. Portion of egg capsules, natural 

 size; they decrease in si/-- toward one end of the 

 string, which is frequently over thirty centimeters 

 long. (Drawn from specimen.) 



Order 2. Opisthobranchia 



The Opisthobranchia (Gr. oTriadios, behind, and /3pdyx ia , 

 gills) are all marine and include the sea slugs and the sea 



