124 



SYSTEMATIC ZOOLOGY 





and they probably gain entrance when the pigs eat other infected 

 animals, such as rats, or the viscera of other trichinous pigs. 



CLASS II. ACANTHOCEPHALA 



The Acanthocephala (Gr. atcdvOa, spine, and KtfyaXr), head) 

 are all parasitic, for the most part in the digestive tract of fishes, 

 and differ from the Nematoda in two striking 

 respects as well as in several details of struc- 



lm 



rnv 



9 rt P 



Fig; 115. A, Echinorhynchus gigas, natural size; female from pig. B, Echinorhynchus 

 lesini/ormis, magnified ; male from the frog, b, bursa; cgl, cement glands ; ^///.genital 

 pore; Im, lemnisci; >!, neck; p and pr, proboscis; r.nt, retractor muscle of proboscis; 

 s.lg, suspensory ligament; /, testis; v, vessel. (After Parker and Haswell.) 



ture. In the first place, they have a short, retractile proboscis at 

 the anterior end of the body, provided with numerous chitinous 

 hooks, by means of which the worm attaches itself firmlv to the 

 wall of the intestine of its host ; in the second place, there is no 

 alimentary canal. A body-cavity is present, and the sexes are 

 separate. The reproductive organs are much more complicated 

 than in the Nematoda, and two hosts are necessary for the 

 development of these animals; the larval or immature condition 



