II. TREMATODA 



243 



is present; if but one or two (entoparasites), one is at the anterior end 

 {oral sucker) surrounding the mouth, while a second larger sucker may 

 occur near the mouth (fig. 217), but may be {Ampliislomum) at the poste- 

 rior end. In the ectoparasites there are a pair of anterior suckers near the 

 mouth; at the posterior end a single sucker, or a number of suckers or 

 hooks or both on a sucking disc (fig. 219). 



Other results of parasitism are the weak development of sense organs and 

 brain and development of accessory ganglia near the adhesive organs. Eve 

 spots (two to four) occur occasionally in the ectoparasitic species and in the 

 larvae of the entoparasitic, rarely in their adult condition. The alimentary tract 

 is forked (hg. 21S) and occasionally (fig. 217) has dendritic blind sacs. 



Fig. 217. Fig. 



Fig. 217. — Distomtim hepaticum^ liver fluke (from Boas), vi, caeca of /a, limbs of 

 digestive tract; s^s^, anterior and posterior suckers. 



Fig. 218. — Distomum lanceolatum. c, cirrus, beneath it the opening of the o\dduct; 

 d vitellaria, the ducts leading to the shell gland; g, ganglion; h, testes with ducts to 

 cirrus; / Laurer's canal; 0, ovarj', the shell gland behind it; s', s", anterior and median 

 suckers, the pharjmx and the bifurcated digestive tract leading from s'; u, uterus; w, 

 terminal vesicle of water-vascular (excretory) system. 



To parasitism may also be attributed the great development of the sexual 

 organs, which at maturity fill a great part of the body. Their features may be 

 seen in fig. 218. Two vasa deferentia pass forward from the testes {h), unite 

 and form a seminal vesicle. The terminal portion of the united ducts can be 

 protruded as a penis or cirrus (c) armed with retrorse hooks. It is usually 

 enclosed in a 'cirrus pouch.' The unpaired ovary (0) is very small and pro- 

 duces small eggs, deficient in yolk; hence the paired vitellaria (d) are well 

 developed. The united ducts from these join the oviduct, producing the uterus 

 (u), which receives the eggs, is much convoluted, and empties beside (in some 



