CHAPTER IX 
HIRUDINEA 
Rhynchobdellidae—Pontobdella, Clepsine. 
Hirudinea Gnathobdellidae—Hirudo, Nephelis, 
CHARACTERISTICS.— Animals with a ringed integument, a certain 
number of the annuli or rings corresponding with each 
true segment. A posterior ventral sucker formed by the 
Jusion of some of the posterior somites is present. The 
coclom is much reduced by the ingrowth of connective tissue ; 
it communicates with the vascular system, and contains the 
same fluid. The mouth is anterior, and usually surrounded 
by a sucker; the anus ws dorsal to the posterior sucker. 
Hermaphrodite, with genital openings ventral and median, 
the male in front of the female. Mostly aquatie and blood 
sucking. 
Amongst the Triclade Turbellarians a certain segmentation 
begins to appear, and reaches its highest point in Gunda seg- 
mentata, where the alimentary canal has 25 lateral divert- 
icula, there are 25 testes and 24 pairs of vitellaria, and 
the dorso-ventral muscles are arranged segmentally. In 
the Nemertines we also find every stage from entirely un- 
segmented animals to those in which the alimentary canal, 
the generative organs, the blood-vessels, the muscles, and 
even the proboscis sheath, present a certain repetition of parts 
which is called segmentation. It is usual, but by no means 
universal, for the segmentation of the various organs to agree, 
so that one segment of the body contains a segment or repre- 
sentative of each system of organs. Very often, however, 
some segments of one organ may be suppressed, or fail to 
develope ; and again many segments of one system of organs, as, 
