Invertebrate Gallery of the Indian Museum. 131 



Pteropoda in which the epipodia, or lateral flaps of the 

 middle region of the foot, are in the form of wings or 

 paddles ; and (2) the Siphonopoda in which the lateral 

 flaps of the mesopodium are rolled in and united to form 

 a funnel, or siphon, which also serves primarily for loco- 

 motion. 



a. CEPHALOPODA PTEROPODA. 



[€a0e I69i]. 



The Pteropods or Sea-butterflies are small hermaphro- 

 dite Mollusks usually found in swarms at and near the 

 surface of the open ocean, paddling or flitting by means of 

 their wing-like epipodia. They are divided into two 

 Orders, (i) the Thecosomata, in which there is a mantle 

 and a thin glassy casque-shaped or mask-shaped shell, and 

 (2) the Gymnosomata, in which a mantle and consequently 

 a shell are absent. In the latter group the "arms", or 

 tentacle-like processes of the forefoot, are fairly well 

 developed and carry suckers, resembling, but on a small 

 and scant scale, those of the Cuttle-fishes. The dead shells 

 of the Thecosomata often accumulate in extensive beds at 

 the bottom of the Ocean, forming the deposit known as 

 Pteropod-ooze. The Pteropoda are illustrated in Case 

 i6gB, by shells, models, and enlarged drawings. 



b. CEPHALOPODA SIPHONOPODA. 



[OEasi;© 170^— 171(E], 



To this important sub-Class, which includes the largest 

 and most highly organized members of the Molluscan 

 phylum, — e.g.^ the Nautilus, the Cuttle-fishes, the Squids, 

 and the Octopodes — the name Cephalopoda is commonly 

 confined. 



As in the Pteropods, the fore-part of the foot is fused 

 with the head, and is split up into prehensile processes or 

 *' arms '\ which, however, are not only more numerous — at 

 least eight or ten in number— but are vastly more highly 

 developed. The middle portion of the foot, however, 



I 2 



