504 ZOOLOGY . SECT. 



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. RHYNCHOBDELLIDA. 



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 Clepsine (Glossiphonia), parasitic on Snails, 

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

 Branchellion, on marine Fishes (Fig. 420). 



ORDER 2. ARHYNCHOBDELLIDA. 

 Hirudinea in which there is no proboscis. 



Sub-order 1. Gnaihobdellida. 



Arhynchobdellida in which the mouth is provided with two, 

 or more usually three, toothed jaws. 



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

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

 carnivorous ; Hcemadipsa, the Land-leech. 



Sub-order 2. Herpobdellida. 



Arhynchobdellida in which the mouth is not armed with true 

 jaws. 



This group includes Herpobdella (Nephelis) Trocheta, Orobdella, 

 etc. all fresh- water or terrestrial forms. 



Systematic Position of the Example. 



Hirudo belongs to the family Hirudinidce, of the sub-order 

 Gnathobdellida. 



The presence of jaws places it in the sub-order Gnathobdellida : 

 the possession of ten eyes, and the presence of five rings to all the 

 segments except a few at the anterior and posterior ends, dis- 

 tinguish it as a member of the family Hirudinida? : the genus 

 Hirudo is distinguished by the constant presence of 26 segments 

 and of 102 annuli. 



