410 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 Won-pul- 
monary Gasteropods, which breathe in the water, by means of branchiz 
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 dicecious, 
or in separate sexes, and the nervous centres are widely separated in 
the body. The first family of this order is Atlantide, of which 
the types are the fossil genus Bellerophon and the recent genus 
Atlanta. The second family is Firolidze, the typical genera of which 
are Carinaria and Firola, The shellin 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 branchize remind us of the spines with which many 
lepidopterous larvee are covered. 
The first family of the Nudibranchiata section is Elysiade : 
typical genera, Limapontia and Elysia. 
The second is PAylirhoide; typical genus, Phyllirhoe. 
The third is #olide@; typical genera, Glaucus, Alderia, and Aolis. 
The fourth is Zritoniade ; typical genera, Scyllea and Tritonia. 
The fifth is Doride, typical genera, Idalia, Triopa, and Doris. 
The first family of the second section, Zecttbranchiata, is Phyll- 
diade ; typical genera, Diphyllidia and Phyllidia. 
The second family is P/eurobranchide ; typical genera, Umbrella 
and Pleurobranchus, 
The third family is Aplystade; typical genera, Dolabella and 
Aplysia. 
The fourth family is Buide; typical genera, Scaphander, Acera, 
Cylichna, Amphisphyra, and Bulla. 
The fifth family is Zornatellide ; typical genera, Tornatina and 
Tornatella. 
In the order of the Opistho-branchiate Molluscs we reach a group 
