182 MfJTAZOAN PHYLA 



Acanthocephala. Such a worm has a protrusible proboscis covered with 

 hooks at the anterior end of the body (Fig. 89). By means of the pro- 

 boscis it becomes attached to the intestinal wall of the host, which is 

 always a vertebrate. The worm lacks an alimentary canal and absorbs 

 digested food in the intestine of the animal in which it lives. All classes 

 of vertebrates are parasitized by these worms, but they occur more com- 

 monly in the fishes, in turtles, and in birds like the herons and bitterns. 



Proboscis „ / — , 



/ Bra//^ Testes 



Prc^stcfte gr/crnc^s 



M^!^^>r^^^y^:^>~. Bursa 



Probosc/s 

 shecr-^h 



//iverfof musc/es Refrt?icfor musc/es <~^=^ 



Fig. 89. — Echinorhynchus ranae (Schrank), showing general organization of a young 

 male. (From VanCleave, '^ Invertebrate Zoology," by the courtesy of McGraw-Hill Book 

 Company, Inc.) 



The intermediate hosts of many of them are Crustacea and aquatic 

 insects. A spiny-headed worm which infects the pigs in the southern 

 parts of this country has as its intermediate host the larva of the June 

 beetle, known as the white grub, which is rooted up and eaten by the pig. 

 216. Econoniic Importance. — It is clear that some of the parasitic 

 nematodes are of great economic importance on account of the production 

 of disease both in man and in domestic animals. Among the thread- 

 worms are also some which are injurious to cultivated plants. Others 

 are distinctly beneficial, feeding upon the injurious forms or destroying 

 various injurious microorganisms. 



