166 ANNELIDA [CH. X 



Order 2. Polychaeta. 



Chaetopoda which have the chaetae arranged in bundles of 

 some size, almost always borne on conspicuous lateral out- 

 growths of the body termed parapodia : the prostomium has, 

 as a rule, tactile organs, known as tentacles and palps : there 

 are no localised generative organs, ova and spermatozoa being 

 developed from wide stretches of the coelomic wall ; the sexes 

 are separate : in the development a well-marked larval stage 

 occurs : with few exceptions the group is marine. 



Ex. Nereis. 



Order 3. Hirudinea. 



Chaetopoda in which chaetae and parapodia are absent and 

 which move by means of a muscular sucker developed on the 

 under surface of the posterior segments : there are no tentacles 

 and the mouth acts as an anterior sucker : the coelom is 

 reduced to capsules surrounding the genital cells and to a few 

 narrow channels : the animals are hermaphrodite, and the 

 genital pores single and median : the members of this order 

 live on the juices of other animals, and there are both fresh- 

 water and marine species : development is entirely embryonic. 



Sub-order 1. Acanthobdellidae. . ; 



Leeches which retain a few pairs of chaetae and have spacious 

 coelomic cavities divided by septa. 



Ex. Acanthobdella. 



Sub-order 2. Rhynchobdellidae. 



Leeches which have lost all chaetae and in which the coelom 

 is reduced to a series of narrow channels. The stomodaeum 

 can be everted and is termed a proboscis. 



Ex. Clepsine (Glossipkonia). 



Sub-order 3. Gnathobdellidae. 



Leeches which have lost chaetae and have a reduced coelom. 

 The stomodaeum is not eversible but forms a sucker provided 

 with three thickened muscular ridges covered with rough cuticle 

 which act as jaws. 



Ex. Hirudo, NepMis. 





