TYPE PLATYHBLMINTHE8. 



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 of Tke- 

 MATODK, I'riaiomum molcB (after 

 Lang). 



Fig. 78. — Excretory System 

 OF Trematodb, Distomum 

 diver gens (after Fbaipont). 

 A, euliie system; B, termiual 

 fuunels. 

 / = funuel. 

 fl = flame of cilia. 

 n = main truuk. 

 ph = pbaiyngeal bulb. 

 s = anterior sucker. 

 U = termiual cell. 

 «s = ventral sucker. 

 •Bt = coutraclile vesicle. 



^ 



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

 the Platyhelminths, 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 

 {vt) 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 



