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CEPHALOUS MOLLUSCA. 



CHAPTEE XIII. 



GASTEROPODA. 



WE shall now consider the Gasteropoda, which is divided into four 

 orders. Firstly, Nucleobranchiata, animals which float on the surface 

 of the ocean : they are Diaecious, or in separate sexes, and the nervous 

 system is widely distributed in the body, the shell, in Carinaria, for 

 instance, covering only a very small portion of the body. The first 

 family of this order is Atlantidse, of which the types are the fossil 

 Bellerophon and the recent Atlanta. 



The second family is Firolidae, the types of which are Carinaria and 

 Firola. Carinaria or glass nautilus is shaped like the bonnet-cap shell, 

 Pileopsis. It is as transparent as glass; and although now very 

 common, was formerly one of the most highly-prized shells by collec- 

 tors. The second order of Gasteropoda is Opistho-Branchiata, and is 

 divided into two sections, the Nudibranchiata, and the Tectibran- 

 chiata. The Nudibranchiata have no shell except in the larva state ; 

 they mostly live at the bottom of the sea on rocky shores, but a small 

 number swim on the surface. They are remarkable for their variety 

 of form and vivid colouring, being the most beautiful of all molluscous 

 animals ; they may truly be called the caterpillars of the sea, for their 

 branchiae remind us of the spines with which many lepidopterous 

 larvae are covered. 



The first family is Elysiadae, types Limapontia and Elysia. 



The second is Phyllirhoidae, type Phyllirhoe. 



