272 



PL A THELMINTHES. 



Other results of parasitism, are the weak development of sense 

 organs and brain and a tendency to development of accessory 

 ganglia near the adhesive organs. Eye 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 (fig. 233) and occasionally (fig. 232) has dendritic blind 

 sacs. 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 maybe seen in fig. 233. Two vasa deferentia pass for- 



s..i 



FIG. 232. FIG. 233. 



FIG. 233. Distomum hepaticum, liver fluke. (From Boas.) ?n, cseca of <, limbs of 

 digestive tract ; Sj.s 2 , anterior and posterior suckers. 



FIG. 233. Distomum Innceolatum. c, cirrus, beneath it the opening of the oviduct ; d, 

 vitellaria, the ducts leading to the shell gland ; g, ganglion; /i, testes with ducts 

 to cirrus ; I, Laurer's canal ; o, ovary, the shell gland behind it ; s', s", anterior 

 and median suckers, the pharynx and the bifurcated digestive tract leading 

 from s'; u, uterus ; w, terminal vesicle of water-vascular (excretory) system. 



ward from the testes (A), unite and form a seminal vesicle. The 

 terminal portion of the united ducts can be protruded as a penis 

 or cirrus, armed with retrorse hooks. It is usually enclosed in a 

 ( cirrus pouch/ The ovary (o) is very small and produces small 

 eggs, deficient in yolk; hence the vitellaria (d) are well developed. 

 The ducts from these unite with the oviduct, producing the uterus 

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

 (in some species in a common antrum with) the male sexual 

 opening. The first part of the uterus is called the ootype because 

 here the eggs and yolk cells are formed into eggs (fig. 227) and 



