PTEROPODA AND GASTKOPODA. 535 



LECTURE XXII. 



PTEROPODA AND GASTROPODA, 



Although the Acephalous MoUusea 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 Lamellibranchs 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-butter- 

 flies. Amongst the Mollusca provided with a distinct head locomo- 

 tion is the rule. Lithedaphiis, indeed, is fettered by its calcareous 

 operculum to the rock on which it grows, and Magilus becomes im- 

 moveably sunk in its coral bed ; but these are rare exceptions. 

 Amongst the numerous free and locomotive Encephala, 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 pro- 

 vided with two fin-like muscular expansions, attached to the sides of 

 the neck, which, from their resemblance to wings, suggested to 

 Cuvier * the name Pteropoda for this small but well marked group 

 of MoUusks with heads. 



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 weU as at the 

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

 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 Limacina constitute, in the northern seas, the principal 

 article of food of the great whales. 



Those Pteropoda which have symmetrical shells composed of two 

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



* CCCXXI. 



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