THE BOniNORTNCHUS. 



165 



Order 1. Acanthocepliali. — These are aberrant Nematode 

 worms (sometimes referred to a separate class), without any 

 mouth or digestive tract, but with an extensible spiny beak, 

 living by imbibition of the fluids of the alimentary canal of 

 their host. 



The thick subcuticula is penetrated by a network of ves- 

 sels, whose trunks form two oval bodies of unknown use 

 called UmnUci, whicli hang down free in the body-cavity. 

 The sexes of Ecliinorhynchus are distinct. The eggs are 

 usually spindle-shaped. The embryo develops in the body 

 of the parent worm, and is surrounded by several membranes, 

 with a circle of hooks arranged bilaterally around the mouth. 

 The embryo contains an oval mass of nuclei, being the ru- 



Fig. 112. — Echincn'yvchus, head retracted and in the second figure extraded ; mag- 

 nifiea. a, oval pore ; b &, protractile muscles ; c c, Jemnieci. —After Owen. 



diments of an intestinal canal. Finally it joasses into 

 some crustacean or insect, in whose body it becomes so far 

 developed, that when its host is swallowed by some vertebrate, 

 such as a fish, the embryo is liberated in the intestines of the 

 second (vertebrate) host and soon attains sexual maturity. 

 Nearly a hundred species are known. 



Echinorhynclius (jigas, the female of which is 50f centime- 

 tres (20 inches) in length, lives in the small intestine of the 

 pig. Its eggs pass out, becoming scattered on the ground, 

 where they are eaten by the white grub or larva of the Euro- 

 pean cockchafer. The egg-membranes burst in the stomach 

 of the grub, and the embryos thus liberated penetrate, by 



