Phylum Nemathelminthes and Related Phyla 443 



While these worms possess a pseudocoel, it is largely filled with 

 parenchymatous tissue. They lack an excretory system, and the diges- 

 tive system though complete in immature worms degenerates at both 

 ends in the adult. There is a nerve ring around the esophagus which 

 connects to a single midventral nerve cord. There are a few sensory 

 bristles and eyespots in the head region. The body wall is similar to 

 that of the other pseudocoelomates. The sexes are separate, and the male 

 may be distinguished by the coiled posterior end. Each sex has two 

 gonads located in the pseudocoel. Paired reproductive ducts carry the 

 gametes to the cloaca. 



Females lay the eggs in long strings usually about aquatic vegeta- 

 tion. Each egg hatches into a small larva which possesses a stylette 

 at the anterior end. These larvae swim about until they come in contact 

 with some suitable insect, usually a grasshopper or cricket which 

 has come to drink. In this host, the larva grows in size, gradually 

 filling most of the internal cavity. When the cricket or grasshopper 

 goes to the water to drink — a desire which seems to be encouraged 

 by the presence of its parasite — the worm escapes and the adult gordi- 

 acean emerges. 



PSEUDOCOELOMATE PHYLA OF MORE DISTANT 

 RELATIONSHIP TO THE NEMATODES 



Three other pseudocoelomate phyla, the Acanthocephala, the En- 

 toprocta, and the Priapulida, all possess characteristics similar to those 

 of the Nemathelminthes ; however, other diverging characteristics dem- 

 onstrate the remoteness of this relationship. 



The Phylum Acanthocephala. — The members of this phylum 

 are entirely parasitic, with the adult stage being found in the intestine 

 of various vertebrates and the larval period in various arthropods 

 (Fig. 144). Many have complex life histories involving several larval 

 changes. 



In common with the other pseudoceolomates, the acanthocephalans 

 possess a pseudocoel, a protonephridial excretory system which may 

 be lacking in some species, a cuticle, a syncytial epidermis, and sub- 

 dermal musculature. They differ conspicuously in the entire absence 

 of a digestive system and by their possession of an anterior, retractible 

 proboscis which is covered with hooks. A muscular proboscis recep- 

 tacle extends into the pseudocoel. Closely associated with this are 



