TKEMATODA. 



317 



The excretory apparatus consists of two large lateral trunks and 

 a network of fine vessels permeating the tissues and beginning with 

 small ciliated lobules. The two large trunks open into a common 

 contractile vesicle, which opens to the exterior at the posterior end 

 of the body (fig. 253, Ep). The excretory system contains a watery 

 fluid with granular concretions. This fluid is probably an excretory 

 product, corresponding to the urine of higher animals. 



The nervous system consists of a double ganglion lying above the 

 oesophagus, and from it several small nerves and two posteriorly 

 directed lateral trunks are said to be given off. Eye sj)ots with 

 refractive bodies are sometimes present in the larva? during their 

 migrations. Locomotion is effected 

 by the dermal muscular system and 

 the organs of attachment, viz., the 

 suckers and hooks, which present 

 numerous modifications in number, 

 form, and arrangement. In general, 

 the size and development of these 

 organs are related to the endo- 

 parasitic or ecto-parasitic mode of 

 life. In the endo-parasitic Trema- 

 todes they are less developed, and 

 usually consist of the oral sucker and 

 a second larger sucker on the ven- 

 tral surface, either near the mouth, 

 as in Distomum, or at the opposite 

 pole of the body (AmpJiistomu'ni). 

 This large sucker may, however, be 

 absent (Monostomuni). The ecto- 

 parasitic Polystomea, on the other 

 hand, are distinguished by a much 

 more powerful armature, for besides 

 two smaller suckers at the sides of the mouth, they possess one or 

 more large suckers at the posterior end of the body (fig. 258), which, 

 moreover, may be supported by rods of chitin. There are often 

 in addition chitinous hooks, and very frequently two larger hooks 

 among the posterior suckers in the middle line (//). 



The Trematoda are mostly hermaphrodite. As a rule, the male and 

 female generative openings lie side by side, or one behind the other, 

 not far from the middle line of the ventral surface, near the anterior 

 end of the body (fig. 254). The male opening leads into a sac, the 



FIG. 253. Young Ditfomum (after La 

 Valette). Ex, trunk of the excretory 

 (water vascular) system ; Ep, excre- 

 tory pore ; O, mouth with sucker ; 

 S, sucker in the middle of the ventral 

 surface ; P, pharynx ; D, forked' in- 

 testine. 



