438 The Animal Kingdom 



The adult female is found just under the surface of the skin when 

 it is ready to deposit its young. Previous to that, it is in the deeper 

 tissues of the skin. The exact position of the worm varies with the 

 individual. The worm makes a tiny hole in the skin, then excretes an 

 irritating substance which forms a blister. This breaks, and when 

 the ulcer thus revealed is washed with water, a milky substance con- 

 taining many embryos emerges. The opening in the worm closes as the 

 uterus dries after each bath. After several extrusions, the worm dies. 

 The embryos which are now free in the water swim about until a 

 suitable host is encountered. This host is a small crustacean, Cyclops. 

 The worm burrows into the crustacean, and after a period of develop- 

 ment, it is ready to infect a new individual who inadvertently drinks 

 the water complete with Cyclops. 



In these complex life cycles, one is constantly impressed with the 

 neatness of the way the details of the life cycle fit with the habits 

 and ecology of the various hosts. Only a long period of association 

 and evolution can account for these, and human carelessness and custom 

 contribute much to their continuing success. 



MINOR PHYLA RELATED TO THE NEMATODES 



Closely related to the Nemathelminthes are a number of smaller 

 phyla whose members show many characteristics in common with the 

 nematodes. Some of these characteristics are such important features 

 as the possession of a pseudocoel and cuticle, and the lack of a 

 definite muscular wall in the digestive tract. These animals include the 

 members of the phyla Rotifera, Gastrotricha, Kinorhyncha, and Nema- 

 tomorplta, as well as the Nemathelminthes. The common possession of 

 these important characteristics by these five phyla has led some zoolo- 

 gists to group them as a single phylum, the Aschelminthes. Each of 

 these phyla as recognized here then is considered as a class ; however, 

 this is simply a matter of interpretation. Inasmuch as each possesses 

 distinctive features, it seems best to consider them as separate though 

 related phyla. 



The Phylum Rotifera. — Like the protozoans, the rotifers may 

 be found in nearly any body of water, no matter whether large or 

 small. Most dwell in fresh waters although there are a few marine 

 species and a small number of parasitic forms. They are easily recog- 

 nized by their possession of wheel-like circles of cilia (thus the name 

 Rotifera) at the oral end. They may swim freely, be attached, or 



