Acanthocephale. Hook-Headed. 243 



head flat in place of globular or tetragonal, and second by the 

 presence on each lateral margin of the head of a depression or 

 groove representing a sucking disc. The generative organs are 

 much less developed than in the true tape-worms, and they pro- 

 duce relatively fewer ova. In experiments the taenia form has 

 appeared to be developed direct from the egg in the intestine of 

 its host, and as the parasite thus escapes the dangers of destruc- 

 tion to which the true tape-worm is exposed through failure to 

 find its appropriate host in its larval form and again in its 

 mature state, it has a compensation for the paucity of ova. The 

 bothriocephala are mostly the para.sites of fish, but a certain 

 number find hosts in man and fish eating carnivora. 



Trematodes. 

 The trematodes have been described as liver parasites. 



ACANTHOCEPHALE. HOOK-HEADED. {Acantha a thorn, 



cephale head). 



Cylindroid worms ; no true digestive apparatus ; males and females in 

 separate individuals ; larval stage in separate genus ; protractile conical pro- 

 boscis armed with hooks. 



These are cylindroid worms, which differ, however, from the 

 true round worms* (nematoids),by the absence of any distinct di- 

 gestive apparatus, nourishment taking place, by imbibition through 

 the integument. They resemble the round worms in having the 

 sexes in different individuals. The males have two ovoid testicles 

 the efferent ducts of which unite in a common canal and single 

 penis. The female has single ovary and long evident opening at 

 the posterior end of the body. I/ike as in the taenia, the embryos 

 of the acanthocephala have to pass through a larval stage, encysted 

 in the body of another animal (invertebrate), which being de- 

 voured by a suitable fish, bird or mammal, affords the opportunity 

 for the development of the parasite to the mature condition. 



The larval stage has been observed in a mollusc and in differ- 

 ent crustaceans. 



The most characteristic feature of the acanthocephala, is the 

 protractile conical proboscis surrounded by several rows of re- 

 curved hooks by which the worm attaches itself to the mucosa of 

 its host. 



