PHYLUM COELHELMINTHES (ANNELIDA) 155 



groups. This same author suggests that leeches may be oUgo- 

 chaetes modified for predatory habits. 



The digestive system does not He loosely in a spacious coelom as 

 does that of the Chaetopoda, for the coelom is largely encroached 

 upon by a parenchyma which leaves a system of blood-filled 

 sinuses in addition to the definite circulatory system. The 

 terminal buccal chamber opens into a muscular pharynx which, 

 in turn, leads by way of an esophagus into a stomach or crop. 

 This latter may be either a straight tube or may give rise to 

 from one to twenty pairs of lateral diverticula before emptying 

 into the intestine. A small rectum leads to the anus on the dorsal 

 surface near the posterior extremity. 



A met anephri dial system similar to that found in the Oligo- 

 chaeta serves for excretion. Usually, not more than seventeen 

 pairs of nephridia occur, for they are lacking from the somites of 

 both extremities and from some of the clitellar somites. The 

 central nervous system consists of a brain and a ventral chain of 

 ganglia of which there are frequently twenty-three. 



Leeches are hermaphroditic. Fertilization occurs either by 

 reciprocal copulation or through the implantation of spermato- 

 phores on the skin. Spermatozoa escape from these and pene- 

 trate the tissues to the ovarian sacs where fertilization occurs. 

 Development from the egg is direct. In many leeches a cocoon 

 is formed by the clitellum for sheltering the eggs and young 

 as in the oligochaetes. In some species, however, the eggs are 

 carried on the ventral surface of the parent's body and even the 

 young remain attached there for some time after hatching. 



Hirudinea get their vernacular name of bloodsuckers from 

 the fact that many species are permanent or temporary ecto- 

 parasites on the bodies of other animals. Many species, and 

 especially the young, are predaceous in habit, feeding upon other 

 small organisms, and resort to the parasitic habit only when 

 opportunity is offered. Most leeches are inhabitants of fresh 

 water, though some are marine and a few are terrestrial. Those 

 which feed upon blood have either three jaws supplied with sharp 

 teeth (Gnathobdellida) which lance the skin of the host or a 

 conical proboscis evertible from the pharynx (Rhynchobdellida) 

 for piercing. Glandular secretions from the leech hinder coagu- 

 lation of the blood and render its wounds difficult to staunch. 



Two orders are commonly recognized, the Rhynchobdellida 

 and the Gnathobdellida. Of these, the former are jawless 



