CHAPTEE IX 



HIRUDINEA 



Hirudinea -f S^^yi^ohobdellidae — Pontobdella, Clepsine. 

 \ Gnathobdellidae — Hirudo, Nephelis. 



Chaeacteristics. — Animals with a ringed integv/ment, a certain 

 numier of the annuli or rings corresponding with each 

 true segment. A posterior ventral sucker formed hy the 

 fusion of some of the posterior somites is present. The 

 coelom is much reduced hy the ingrowth of connective tissue ; 

 it communicates with the vascular system, and contains the 

 same fluid. The mouth is anterior, and usually surrounded 

 hy a sucker; the anus is dorsal to the posterior sucker, 

 hermaphrodite, with genital openings ventral and median, 

 the male in front of the female. Mostly aquatic and hlood 

 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 agaia many segments of one system of organs, as, 



