TYPE PLATYHELMINTHES. 



145 



the ventral surface of the body, two on each side of the mid- 

 dle line, the other two having a more dorsal position, while 

 when only two are present they correspond to the two more 

 median ventral nerves of this 

 arrangement. Sense-organs are 

 but feebly developed as a rule, 

 especially among the endopar- 

 asitic forms, but in some ecto- 

 parasites eyes are present con- 

 sisting usually of four spots of 

 pigment seated upon the brain- 

 ganglion and sometimes provid- 

 ed with a lens-like structure. 



FIG. 77. NERVOUS SYSTEM OP TRE- 

 MATODE, Trixtomum molce (after 

 LANG). 



A 



FIG. 78. EXCRETORY SYSTEM 

 OP TREMATODE, Distomum 

 divergens (after FRAIPONT). 

 A, eutire system; B, terminal 



funnels. 

 / = funnel. 

 fl = flame of cilia. 

 n main trunk. 

 ph pharyngeal bulb. 

 s = tiiiterior sucker. 

 tc = terminal cell. 

 vs = ventral sucker. 

 tit = contractile vesicle. 



The excretory apparatus (Fig. 78) consists, as is usual in 

 the Platyhelininths, of two longitudinal, more or less irregu- 

 larly twisted tubes (n) from which arise the funnel-bearing 

 branches (/). A peculiarity of the Trematodes is, however, 

 the union of the two longitudinal tubes in a terminal vesicle 

 (vf) which opens to the exterior at the hinder end of the body 

 by a single pore. 



The reproductive system is exceedingly complicated, though 

 essentially similar to that of the higher Turbellaria. It opens 



