160 



COLLEGE ZOOLOGY 



least 35 other kinds of insects, including 

 the corn earworm, boll weevil, pink boll- 

 worm, vegetable weevil, cabbage worm, and 

 white fringed beetle. 



Origin and relations of 

 tFie Nemathelminthes 



The Nemathelminthes seem to occupy a 

 rather isolated position in the animal series. 

 In many respects they resemble the platy- 

 helminthes; they are unsegmented and pos- 

 sess excretory canals but no flame cells. 

 There is an absence of cilia. In the nema- 

 todes, we encounter, for the first time, ani- 

 mals with two openings in the digestive 

 tract, a mouth and an anus. The sexes are 

 usually separate, whereas in the Platyhel- 

 minthes hermaphroditism is the rule. The 

 parasitic roundworms evolved from free- 

 living roundworms. 



crickets to the water is problematical. How- 

 ever, one investigator discovered that when 

 crickets found near the edge of a lake were 

 placed in the water, a horsehair worm in- 

 variably escaped from the cricket's body in 

 less than a minute, but crickets collected 

 100 to 300 feet from the lake did not yield 

 worms. Paragordius varius is a horsehair 

 worm common in North America (Fig. 

 81). 



The Nematomorpha are like the Nema- 

 thelminthes because of body form, presence 

 of cuticle, simple musculature, and absence 

 of segmentation. However, in Nematomor- 

 pha there are important differences, such 

 as a body cavity which is nearly filled with 

 parenchyma, a degenerate digestive tract in 

 adults, a single nerve cord, and a cloaca. 

 No circulatory, respiratory, or excretory or- 

 gans are present in the Nematomorpha. The 

 physiology of this group is not well under- 

 stood. 



PHYLUM NEMATOMORPHA 

 OR HORSEHAIR WORMS 



The horsehair worms 



The name "horsehair" comes from a 

 popular superstition, not yet dead, that this 

 roundworm develops from horsehairs that 

 fall into water. Actually their life cycle is 

 as follows. The adults live in fresh water 

 where the eggs are laid. The larv^ae that 

 hatch from the eggs penetrate the body of 

 some aquatic insect larva. These worms 

 migrate to the body cavity and continue to 

 develop until they escape as young adults 

 or juveniles. They have been found in dif- 

 ferent stages of development in the body 

 cavities of beetles, crickets, and grass- 

 hoppers, all of which are terrestrial, suggest- 

 ing that these land forms may eat aquatic 

 insects containing the minute roundworm 

 larvae. Infested crickets appear to migrate to 

 the edge of water, and, if caught by a wave, 

 Paragordius emerges from their bodies 

 within 20 to 50 seconds. What brings the 



PHYLUM ACANTHOCEPHALA 

 (SPINY-HEADED WORMS) 



The spiny-headed worms 



These peculiar parasitic worms (Fig. 85) 

 belong to the phylum Acanthocephala, a 

 name that means spiny-headed and refers 

 to a retractile proboscis armed with rows of 

 recurved hooks. They live in the intestine 

 of vertebrates and are attached to the wall 

 by a protrusible proboscis covered with re- 

 curved hooks; they vary in length from less 

 than an inch to more than a foot. The body 

 of most species is elongate, flattened, and 

 capable of extension. No digestive tract is 

 present at any stage in their life cycle, food 

 being absorbed directly from the host's in- 

 testine. The sexes are separate, and the re- 

 productive systems are complex. Species 

 have been reported in the United States 

 from fish, turtles, birds, rats, mice, pigs, 

 squirrels, dogs, and man. 



The Acanthocephala differ from the Nem- 



