278 PLATHELMINTHES. 



America. Other species occur encysted in man, two (D. ophthalmobius and 

 Monostomum lentis) in the capsule of the lens and in the lens itself. The 

 genus Amphistomum is common in the intestine of Ungulates, one species, 

 A. hominis, occurring in man. With few exceptions the adult stages of 

 all Distomes occur in vertebrates, the larval stages in molluscs. Aquatic 

 birds are very apt to be infested with them, and " it may be of interest to 

 gourmets to know that the trail of a woodcock largely consists of distomic 

 Trematodes." 



Class III. Cestoda. 



The majority of the cestodes, and especially those of the human 

 intestine, are distinguished from the similarly entoparasitic trema- 

 todes in a striking manner. But the boundaries between the two 

 groups disappear in certain forms like Archigetes, Caryophyllceus, 

 and AmpUilina, parasitic in lower vertebrates or invertebrates and 

 which are now assigned to the trematodes, now to the cestodes. 

 The most important character of the cestodes is that as a result of 

 their parasitic life they have lost the last traces of an alimentary 

 canal, and are nourished by the juices or the partially digested 

 food of the host, since the fluid nourishment is taken in through 

 the skin into the body parenchyma. It is a disputed question 

 whether the cuticula of the surface is penetrated with pores for 

 this purpose. 



Two other characters are so striking that they are among the 

 first thought of. (1) The differentiation of two developmental 

 stages, the bladder worm, or cysticercus, living chiefly in paren- 

 chymatous organs (muscles, liver, brain), and the sexually mature 

 animal, living as a parasite in the alimentary tract; (2) the division 

 of the body of the adult into different parts, the head or scolex, 

 and following this a series of joints or proglottids. Since this last 

 feature holds for all human tapeworms and hence for the best 

 known species, the following description begins with these typical 

 forms. 



The sexually mature tapeworm or strobila (fig. 238) consists of 

 a single scolex in front, and behind this follow in a single row the 

 proglottids. The number of these last varies from smaller forms 

 (Tcenia ccliinococcus, fig. 252) with three or four to several hun- 

 dreds or even several thousands, a fact which speaks for the enor- 

 mous size of some species. The proglottids are derivatives of the 

 scolex, from the hinder end of which they become separated by a 

 kind of budding. This explains the well-known fact that the body 

 is not rid of the tapeworm, so long as the head remains in the 

 host. It also explains the peculiar shape of the worm, which is 



