210 



MESOZOA THROUGH ENTOPROCTA: 



Figure 12.14 Cavity worm types: A, a rotifer, Philodina; B, a horsehair worm, Gordius; C, a 

 roundworm, Ascaris, female above and male below; D, a fork-toiled worm, Choetono/us; E, a 

 spiny-crowned worm, Echinoderes; F, a club worm, Priapulus. 



rounded anterior end, and slightly tapered pos- 

 terior end; external resistant surface opaque and of- 

 ten colored; adults in fresh water, often ponds, rain 

 puddles, or drinking troughs; larvae are insect para- 

 sites. 



Because the adults superficially resemble the tail 

 hairs of horses, a superstition, and hence the common 

 name of the class, resulted. Some people actually be- 

 lieve that these worms are horsehairs that have come 

 to life, probably because the adult worms were often 

 found in drinking troughs used by horses. The actual 

 story might not seem much less remarkable. The fe- 

 male worm deposits strings of eggs on objects and 

 each egg hatches into a microscopic larva which is 

 thought to enter an aquatic larvae of its host or to 

 encyst upon vegetation. In the latter possibility, if the 

 cyst is eaten by certain insects, usually grasshoppers 

 or crickets, the larva emerges from the cyst and 

 transforms to an adult; no matter how the larval 



worm may enter an insect, the larval worm then be- 

 comes an adult horsehair worm. Finally, if the insect 

 falls into water, the adult horsehair worm emerges. 

 These "ifs" may seem unlikely to occur, but they do. 

 In fact, the completion of life cycles of most parasites 

 is based on many unlikely "ifs" that do occur be- 

 cause great numbers of eggs and larvae are produced 

 by the adults. Of thousands of eggs or larvae, only 

 one may complete its life cycle. 



CLASS NEMATODA (Roundworms) 



Diagnosis: minute to over a yard long, most 

 small or minute; unsegmented roundworms with 

 slender, cylindrical bodies that often taper toward 

 both ends (not bluntly rounded anteriorly); covered 

 by a resistant cuticle; probably second to the insects 

 in numbers (a few zoologists consider them more 

 numerous); many are free-living in the soil or water, 

 others are plant and animal parasites. 



