SECT. v. AXNELIDA 149 



Annelids. 



The Annelids are the most highly organized of all 

 the worm tribe. They are exceedingly numerous and 

 varied ; some are inhabitants of fresh water, others are 

 terrestrial, but by far the greater number and most 

 highly endowed are marine. They generally have a 

 long, soft, and smooth body, divided or marked by trans- 

 verse rings into a succession of similar segments. In 

 many the first and last segments are alike ; in others 

 the first segment can scarcely be called a head, though 

 it exercises several functions, while in the highest two 

 orders the head is the seat of several senses. On each 

 side of the bodies of the Annelida there are one or two 

 long rows of tufted bristles or feet, which may be re- 

 garded as the earliest form of symmetrical locomotive 

 organs. Most of the Annelids have ocelli or eye-specks, 

 and in many of them the head supports soft cylindrical 

 tentacles, which are obviously organs of touch. These 

 worms are divided into four orders, the Suctorial, Ter- 

 restrial, Tubercular, and Errantia, or Wandering Worms. 5 



The first order consists of Leeches of different kinds : 

 their body is long, slightly segmented, with a suctorial 

 disc at each end. Their skin is smooth, whitish, and 

 translucent ; beneath it are cells filled with brown or 

 greenish matter, and three layers of muscular fibres 

 follow; the first are transverse, the second cross one 

 another diagonally so as to form a network, and the 

 third are longitudinal. The mouth, which occupies the 

 centre of the principal sucking disk, varies in form 

 with the genera. In the common leech it has an en- 

 larged lip, and opens into a short gullet leading into a 

 capacious and singularly complicated stomach, divided 

 by deep constrictions into eleven compartments, the last 



5 According to the system of M. Milne-Edwards, who made the Annulosa 

 a particular object of investigation. 



