PHYLUM ANNULATA 



475 



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

 system consists of a brain united by short oesophageal 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. RHYNCHOBDELLIDA. 



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

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



2.Clebsine 



3. Bra n c h e 1 1 io n 



I.Ponl-obdella 



FIG. 379. Three Rhynchobdellida. 6)-. gills; pr. everted proboscis. (1, after Bourne, 2 and 3, 



after Cuvier.J 



