516 ZOOLOGY SECT. 



lies in the anterior sucker, the anus is usually dorsal and imme- 

 diately in front of the posterior sucker. The crelome is always 

 more or less obliterated by connective-tissue, and is rejjresented 

 by sinuses of varying dimensions which contain blood. True 

 blood-vessels, with muscular walls, are also present. The nervous 

 system consists of a brain united by short ossophageal connectives 

 to a ganglionated ventral nerve-cord. The excretory organs are 

 segmentally arranged nephridia. The sexes are united, the 

 testes numerous and usually segmentally arranged, the ovaries 

 a single pair. The testes have the form of sacs, and discharge 

 their products internally : the ovaries either have a similar struc- 

 ture or are band-like and enclosed in ovarian sacs, into which the 

 ova are set free. The penis and the vagina are unpaired, and open 

 by median apertures, the male anterior to the female, on the 

 ventral surface of the body. Development is usually direct, i.e. 

 unaccompanied by a metamorphosis. Leeches are either free- 

 living, or are permanently or intermittently parasitic : they inhabit 

 either the land, fresh-water, or the sea. 



The class is divided into the following two orders : — 



Order 1. — Rhvnghobdellida. 



Hirudinea in which the anterior part of the body can be pro- 

 truded and retracted so as to form a proboscis or introvert. 



This order includes C'lepsine {Glosiiiihonia), parasitic on Snails, 

 Frogs, &c. ; Piscicola, on fresh-water Fishes ; Pontohddla and 

 Branchdlion, on marine Fishes (Fig. 413). 



Order 2. — Arhynchobdellida. 

 Hirudinea in which there is no proboscis. 



Sitb-order 1. — Gnathobddlida. 



Arhynchobdellida in which the mouth is provided with two, 

 or more usually three, toothed jaws. 



This sub-order includes Hiritdo, the common Leech, parasitic 

 on Vertebrata; Aulostoma, the Horse-leech, free-living and 

 carnivorous ; Hwmadipsa, the Land-Leech. 



Sub-07'der 2. — Herpobdellida. 



Arhynchobdellida in which the mouth is not armed with true 

 jaws. 



This group includes Herpohdella (MjJheliv) Trocheta, Orobdella, 

 etc. — all fresh-water or terrestrial forms. 



