GENERAL ZOOLOGY 

 THE ASCHELMINTH BODY PLAN 



As stated at the outset, the phylum Aschelminthes is a very diverse group, 

 and it is difficult to describe a body plan common to members of all the six 

 classes. Certain characteristics are found in all the classes, however, and 

 others occur in two or more. For instance, the nature of the body cavity 

 is a major unifying feature. The condition of the body wall, with its cuticle, 

 simple epidermal epithelium, and muscular layers bounding the pseudocoel 

 externally, is also comparable throughout the phylum. A simple digestive 

 system, marked in almost all groups by a well-developed, muscular pharynx, 

 is common in aschelminths. The reproductive systems are variable, but we 

 should consider the fact that many nematodes, for example, have undergone 

 significant modifications in adaptation to the parasitic way of life. 



OTHER PSEUDOCOELOMATE PHYLA 



The Phylum Acanthocephala. Members of this phylum are usually 

 cylindrical, worm-like animals; without exception, they are found in the adult 

 stages as parasites of the digestive tracts of vertebrates. Their larvae occur 

 in the bodies of invertebrate hosts, usually insects or crustaceans, which thus 

 serve as intermediate hosts in the life cycles of the worms. Adult acantho- 

 cephalans of difTerent species range in length from a small fraction of an inch 

 to about 18 inches. Their most remarkable characteristics are the presence of 

 a proboscis, armed with stout hooks, which has given them the name "thorny- 

 headed worms," and the complete absence in all stages of any trace of a 

 functional digestive tract. The proboscis, which is completely retractable into 

 a sheath, serves to attach the parasite to the intestinal epithelium of the host. 

 The damage inflicted on the gut lining of the host by the hooked proboscides 

 of large numbers of these worms is chiefly responsible for the pathogenicity 

 of the parasites. 



The body of an acanthocephalan may be divided into an anterior presoma, 

 bearing the proboscis, and a posterior trunk (Fig. 12.6). The body wall con- 

 sists of a complex cuticle of several layers, permeated by a system of canals, 

 and two well-developed muscle layers. The body cavity enclosed by the body 

 wall is without a peritoneal lining and thus forms a pseudocoel. This space is 

 occupied by a fluid and contains the excretory organs (compound proto- 

 nephridia) and the reproductive system. 



Acanthocephala are dioecious, and in common with most other parasitic 

 worms they possess highly specialized and complex reproductive systems. 

 Fertilization is internal, and development within the body of the female leads 

 to the production of aconthor larvae (Fig. 12.7) enclosed in thick, resistant 

 shells. These larvae are released by the parasite, reach the exterior with the 

 feces of the host, and are ingested by the arthropod intermediate host. A 

 period of further development within this host is required before the larvae 

 become infective for the final host. 



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