PHYLA NEMATHELMINTHES, NEMATOMORPHA, AND ACANTHOCEPHALA 



161 



Proboscis with hooks 



Lemnisci 



Testes 



Body cavity 



Figure 85. Spiny-headed worm, Acanthocephala: internal structure. These intestinal parasites 

 are found in all classes of vertebrates. Occasionally the worms cause perforation of the gut of 

 the host, resulting in its death. 



athelminthes in the absence of a digestive 

 tract, presence of a proboscis, circular mus- 

 cles, ciliated excretory organs, and certain 

 peculiarities in the reproductive organs. 



CLASSIFICATION OF THE 



NEMATHELMINTHES, 



NEMATOMORPHA, AND 



ACANTHOCEPHALA 



{For reference purposes only) 



Phylum Nemathelminthes (Aschelminthes). 



These roundworms are unsegmented, three- 

 germ-layered (triploblastic), bilaterally sym- 

 metrical animals. The body is cylindrical and 

 elongate with tough resistant cuticle. Digestive 

 tract is a straight tube with mouth and anus 

 at opposite ends of the body. Body wall with 

 longitudinal muscles only; space within body 

 is a pseudocoel. There are no respiratory nor 

 circulatory organs; excretory organs are simple, 

 consisting of one, two, or none. The nerve ring 

 around the esophagus connects with other 

 nerves or cords. No cilia are present and the 

 sexes are usually separate. One class and 5 

 orders are listed here as follows: 



Class 1. Nematoda. With intestine but no 

 proboscis. Body cavity not lined with 



epithelium; gonads continuous with 

 their ducts; no cloaca in female; 

 lateral lines present. 



Order 1. Ascaroidea. Free-living or para- 

 sitic; with three prominent lips. 

 Ex. Ascaris lumbricoides (Fig. 

 78). 



Order 2. Strongyloidea. Parasitic; male 

 with copulatory bursa supported 

 by muscular rays; esophagus 

 club-shaped and without pos- 

 terior bulb. Ex. Necator Ameri- 

 canus (Fig. 82). 



Order 5. Filarioidea. Parasitic; filiform 

 worms without lips; esophagus 

 without bulb, the anterior por- 

 tion being muscular and the 

 posterior glandular. Ex. Wuc/ic- 

 reria bancrofti (Fig. 82). 



Order 4. Dioctophymoidea. Parasitic; 

 moderate to very long nema- 

 todes; mouth without lips, sur- 

 rounded by 6, 12, or 18 papillae; 

 esophagus elongated without 

 bulb. Ex. Dioctophyma renale 

 (Fig. 81). 



Order 5. Trichuroidca or Trichinelloidea. 

 Parasitic. Body filiform anteri- 

 orly; mouth without lips; eso- 

 phageal portion of body more 

 or less distinct; esophagus a 

 cuticular tube embedded in a 



