MUSCULAR SYSTEM. 



The muscular system of the pteropods, like that of many 

 of the floating gasteropods, is generally soft, transparent, and 

 nearly colourless ; so that the disposition of the fibres in the 

 exterior closed mantle, and in the fin-like arms can be per- 

 ceived through the outward pellucid coverings of the body. 

 Having no muscular foot for creeping, their progressive mo- 

 tion depends on the movement of two muscular membranes, 

 unsupported by rays or by cartilage, and extending from the 

 sides of the body. These muscular fins are developed both 

 in the naked and the testaceous forms, they present the most 

 favourable situation for the branchiae which are generally 

 placed on their surface, and they sometimes serve as organs 

 of prehension, embracing the surface of plants, and other 

 objects floating through the sea. The muscular mantle, 

 closed above, forms a large abdominal cavity, and its con- 

 tractions assist in expelling the natural excretions of the vis- 

 cera, or in retracting the whole body within the shell. The 

 tentacula also, and the whole apparatus of the head, are re- 

 tracted and extended by their own muscular fibres, and the 

 oesophagus is sometimes provided with a distinct muscular 

 bulb, like many gasteropods. The annexed figure, (Fig. 6'7,) 

 represents two naked and two testaceous forms of pteropods, 



FIG. 67- 



PART II. 



