41 THE OCEAN WORLD. 



internal organisation in these molluscs, and renders it convenient to 

 divide them into two secondary groups ; namely, Pulmonary Gastero- 

 pods, which breathe in the air, and by a kind of lung, and Non-pul- 

 monary Gasteropods, which breathe in the water, by means of branchiae 

 or gills, or we may consider the Gasteropoda, as they were divided 

 by Milne-Edwards, into four orders. Firstly, NUCLEO-BRANCHIATA, 

 animals which float on the surface of the ocean ; they are dioecious, 

 or in separate sexes, and the nervous centres are widely separated in 

 the body. The first family of this order is Atlantidas, of which 

 the types are the fossil genus Bellerophon and the recent genus 

 Atlanta. The second family is Firolidae, the typical genera of which 

 are Carinaria and Firola. The shell in 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 

 shells most highly-prized by collectors. The shell covers only a 

 very small portion of the body. 



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 larval 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 the 

 variety of their 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 of the Nudibranchiata section is Elysiadce : 

 typical genera, Limapontia and Elysia. 



The second is Fhyllirhoidcz; typical genus, Phyllirhoe. 



The third is sEoltthz; typical genera, Glaucus, Alderia, and yEolis. 



The fourth is Tritoniadcz ; typical genera, Scyllsea and Tritonia. 



The fifth is Dorida; typical genera, Idalia, Triopa, and Doris. 



The first family of the second section, Tectibranchiata, is Phylli- 

 diadce; typical genera, Diphyllidia and Phyllidia. 



The second family \sPleurobranchidce; typical genera, Umbrella 

 and Pleurobranchus. 



The third family is Aplysiadcs; typical genera, Dolabella and 

 Aplysia. 



The fourth family is JSullidce; typical genera, Scaphander, Accra, 

 Cylichna, Amphisphyra, and Bulla. 



The fifth family is Tornatellida ; typical genera, Tornatina and 

 Tornatella. 



In the order of the Opistho-branchiate Molluscs we reach a group 



