272 THE MECHANICAL FUNCTIONS. 



the form of a flat oval disk. This is the case in 

 the JSrpobdella, of which Fig. 130 is an enlarged 

 representation. 



In general, the first of the segments into 

 which the body is divided, contains the prin- 

 cipal organs of sense, and is sufficiently distinct 

 from those which follow to entitle it to the ap- 

 pellation of the head ; while the lengthened pro- 

 longation of the opposite extremity, when such 

 a form is present, may be denominated the 

 tail. 



The rings which encircle the body are con- 

 nected laterally by a looser and more flexible 

 portion of integument, and also by layers of 

 muscular fibres, curiously collected into bands. 

 The muscular flesh of insects, and other animals 

 of this class, differs much from that of the larger 

 animals, being soft and gelatinous in its texture, 

 though endowed with a high degree of irritabi- 

 lity, and contracting with great force. The 

 fibres composing each band are all parallel to 

 one another, and have seldom any tendinous 

 attachments; being generally inserted directly 

 on the parts they are destined to move. Thus 

 the adjacent margins of the rings of worms 

 (as shown in the diagram, Fig. 131) are con- 

 nected together by these muscular bands, which 

 pass transversely from the one to the other, 

 immediately under the skin, and parallel to the 



