///. ANNELIDA: HIEUDINEL 



319 



A third character is the marked flattening of the body in the 

 dorso ventral direction (except in Ichthyobdellidae and a few other 

 forms), the animals thus recalling the flatworms. This may be 

 the result of the very slight development of the coelom. In most 

 leeches there is a body (parenchyma, traversed by longi- 

 tudinal, transverse and dorsoventral muscles in which 

 the organs are immediately imbedded (fig. 290). 



The alimentary tract is provided with paired 

 diverticula (fig. 291), varying in number in different 

 species. Between the last and largest pair of these 

 sacs is the intestine, which opens dorsal to the pos- 

 terior sucker. The jawed and jawless leeches show 

 considerable differences in the pharyngeal region. 



FIG. 290. Transverse section of Hirudo medicinalis. (From Lang.) dm, Zm, rm, dorso- 

 ventral, longitudinal, and circular muscles; vi, vd, w, lateral, dorsal, and ventral 

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

 de^erens; md, midgut; np, nephridial tubule; enp, urinary bladder. 



FIG. 291. Digestive tract of Hirudo medicinalis. (From Lang.) a, oesophagus; 6, in- 

 testine; d,, d a , gastric diverticula. 



In the first there are three jaws in the phaynx, semicircular chitin- 

 ous plates, the free edge of each armed with numerous calcified 

 teeth (fig. 292). To these are attached two muscles, one to retract 

 them, when not in use, into pockets, while the other exserts them 

 and rotates them, causing a triradiate wound from which the blood 

 flows. This bleeding is difficult to staunch, since glands on the 

 lips and between the jaws produce a secretion which hinders the 

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

 process arising from the pharynx can be protruded from the 

 mouth, and serves for wounding and sucking. The vascular 

 system usually contains red blood, and consists, in the Gnatho- 

 bdellidae, of four longitudinal trunks, a dorsal, two lateral, and a 



