PTEROrODA. 291 



LECTUKE XXII. 



PTEROPODA AND GASTEROPODA. 



Although the Acephalous MoUusca are for the most part deprived 

 of the power of locomotion, or have it granted to them in a very low 

 degree, yet some species of Lamellibranchians can swim, and the 

 pectens, from their lively movements in the water, and the vigorous 

 flappings of their brightly tinted valves, have obtained the name of 

 sea- butterflies. Amongst the Mollusca provided with a distinct head, 

 locomotion is the rule not the exception. The Litliedaphus is fet- 

 tered by its calcareous operculum to the rock on which it grows, and the 

 Magilus becomes immoveably sunk in its coral bed. Certain extinct 

 genera of the family Rudistes, allied to the Calyptreidce, were also 

 sedentary ; but all the other encephalous Mollusca are free and loco- 

 motive : some creep, some climb, some swim : a few combine these 

 different powers ; whilst certain small species have no other mode of 

 progression than by floating or swimming on the surface of the 

 ocean. These are provided with two fin-like muscular expansions, 

 attached to the sides of the neck, which, from their resemblance to 

 wings, suggested to Cuvier the name Pterojpoda for this small and 

 lowest organised class of the encephalous Mollusca. 



Some Pteropods are provided with a light and delicate semitrans- 

 parent shell. In the Hyalcea, it resembles a bivalve shell, of which 

 the two valves had been cemented together at the hinge ; leaving a 

 narrow fissure in front and at the sides. In the Cleodora, the two 

 plates of the shell are united together along the sides as well as at the 

 base, leaving an opening only in front. The shell of the Limucina 

 is a cone twisted spirally in one turn and a half. The shell of the 

 Cymbulia is symmetrical, like a boat or slipper, but is cartilaginous. 

 The Clio and Pneumodermon are naked, or without shells. 



All the species of Pteropoda are of small size ; they float in the 

 open sea, often at great distances from any shore, and serve, with the 

 Acalephae, to people the remote tracts of the ocean. In the latitudes 

 suitable to their well-being, the little Pteropoda swarm in incredible 

 numbers, so as to discolour the surface of the sea for leagues ; and 

 the Clio and Limaciyia constitute, in the northern seas, the principal 

 article of food of the great whale. 



Those Pteropoda which have symmetrical shells composed of two 

 plates have one applied to the dorsal, and the other to the ventral, 

 surface of the body, as in the Brachiopoda ; and the two muscular 



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