282 



CCELHELMINTHES 



A third character is the marked dorsoventral flattening of the body 

 (except in Ichthyobdellidie and a few other forms), the animals thus re- 

 calling the flatworms. This may be the result of the very slight develop- 

 ment of the ccelom. In most leeches there is a body parenchyma, traversed 

 by muscles in which the organs are immediately im- 

 bedded (fig. 269). 



The alimentary tract bears paired diverticula (fig. 

 270), varying in number in different species. Between 

 the last and largest pair of these is the intestine, which 

 opens dorsal to the posterior sucker. The jawed and 

 jawless leeches show considerable differences in the 

 pharyngeal region. In the first there are three semi- 

 circular jaws in the pharynx, the free edge of each 

 armed with teeth (fig. 271). To these are attached two 

 muscles, one to'retract them, while the other exserts and 

 rotates them, causing a triradiate wound from which the 

 blood flows. This bleeding is difficult to staunch, since 

 a secretion of glands on the lips and between the jaws f/^ Xf 



jf - ^ 



SC 



1m 



I 



w 



FIG. 269. FIG. 270. 



FIG. 269. Transverse section of Hirudo medicinalis (from Lang), dm, Im, rm, 

 dorsoventral, longitudinal, and cirular muscles; vl, vd, vv, lateral, dorsal, and ventral 

 blood-vessels, the latter surrounding the ventral nerve cord, ni; h, testes; vd, vas deferens; 

 md, midgut; np, nephridial tubule; enp, urinary bladder. 



FIG. 270. Digestive tract of Hirudo medicinalis (after Hatschek). a, rectum 

 and anus; b, last crccal pouches of intestine; d, intestine, opened at d l ; dg, dorsal, Ig, 

 lateral blood vessel; o, oesophagus; sc, nephridia. 



hinders the coagulation of the blood. In the jawless leeches a sharp 

 conical process arising from the pharynx serves for wounding and 

 sucking. The vascular system usually contains red blood, and consists, 

 in the Gnathobdellida?, of four longitudinal trunks, a dorsal, two lateral, 



