2S2 



CCELHELMIXTHES 



A third character is the marked dorsoventral flattening of the body 

 (except in Ichthyobdellid;e and a few other forms), tlie animals thus re- 

 calling lire Ihitworms. Tliis may be the result of llie very slight develop- 

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

 bv muscles in which the organs are immediately im- 

 bedded (fig. 20g). 



The alimentary tract bears paired diverticula (lig. 

 270), varying in number in different species. Between 

 the last and largest pair of tliese is the intestine, wliich 

 opens dorsal to the posterior sucker. The jawed and 

 jawless leeches show considerable differences in the 

 pharvngeal region. In the first tliere 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 tliem, while tlie otlier exserts and . -3^] 

 rotates them, causing a trirachate wound from which the 

 blood Hows. Tills bleeding is difficult to staunch, since 

 a secretion of glands on the lips and between tlie jaws 



W 



Mc 



^^ 



Q 



a 



vr 711 

 Fig. ;6q. 

 Fig. 26q. — Transverse section of Hinido ivcJiiliialis (from T.ant;' 

 dorsoventral, longitudinal, anil cirular muscles; ■:■/, rJ, ;■;', lateral, dors 

 blood-vessels, the latter surrounding the ventral nerve cord, hi; /j, testes; ■; 

 vid, midgut; up, nephridial tubule; cnp, urinary bladder. 



Fig. 270. — Digestive tract of Ilirudo mcdicinalis (after Hatschek). 

 and anus; />, last ca-cal jiouches of intestine; (/, intestine, opened at c/'; dg, 

 lateral blood vessel; o, a-sophagus; sc, nejihridia. 



d uj , hii, r;», 

 .\m\ \entral 

 as deferens; 



(7, reetuni 

 (.lorsal, Ig^ 



liinders the coagulation of the blood. In the jawless k^eches a sharp 

 conical process arising from the pharynx serves for wounding and 

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

 ill the Gnathobdellid;e, of four longitudinal trunks, a dorsal, two lateral, 



