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EVOLUTION OF ANIMALS 



Part V 



Phylum Nematomorpha 



Horsehair Worms. Adult horsehair worms writhe slowly like living wire or 

 he in still coils in the edge-waters of ponds. They used to be common in drink- 

 ing troughs and the wayfarers who saw them added their testimony to the 

 belief that horsehairs "turn to life" after a night in the water. Adult hairworms 

 are from a few millimeters to a yard in length; in shallow water they are easily 

 noticeable; coiled in the body cavity of a freshly killed grasshopper they are 

 spectacular. 



The names of the genera, Gordius and Paragordius, come from the Gordian 

 knot that their coils suggest. 



,Proboscis 



Body cavity 



Cement glantf 



Lemniscus 



ADULT MALE 



Fig. 26.8. Structure of typical spiny-headed worm or Acanthocephala. These 

 worms are parasites of fishes, birds, and mammals in most of the world including 

 the Arctic and Antarctic. They range in size from less than an inch to more than 

 one foot. (Courtesy, Hunter and Hunter: College Zoology. Philadelphia, W. B. 

 Saunders Co., 1949.) 



General Structure — Advance over Flatworms and Nematodes. The 

 body cavity is lined with epithelium and is thus a true coelom. Partitions of 

 loose tissue divide the cavity into compartments. It is not filled with tissue 

 (parenchyma) as the comparable cavity is in nematodes. A single midventral 

 nerve connects with the brain by way of the ring around the esophagus, an 

 arrangement suggesting the one in the earthworm and insects. 



The adult worm is uniformly cylindrical and slender. Its covering of cuticle 

 is very thin but the thickness of the body wall makes the cuticle look opaque. 

 There is no special circulatory, respiratory, or excretory system. The digestive 

 canal is open throughout its length in young worms but may close or degen- 

 erate in adults. The sexes are separate. The eggs are shed from the ovaries 

 into the coelom and then pass into the oviducts which are structurally separate 

 from the ovaries as they are in the vertebrates. 



Life Cycle and Ecology. Several stages in the life history of horsehair 

 worms were discovered many years ago, but the actual life cycle has been 

 learned only recently by controlled experiments in the laboratory, as well as 



