, INTRODUCTION 5 



(6) The Nemathelminthes. The NEMATHELMINTHES are 

 unsegmented round or thread worms. Most of them are parasitic 

 like the round worm, Ascaris (Fig. in), which inhabits the 

 alimentary canal of man, the horse, and many other animals. 

 One dangerous parasite is Trichinella (Fig. 113), which spends 

 part of its life in the muscle of the" hog, and may attack human 

 beings if infected pork is eaten without being sufficiently cooked. 

 Vinegar usually contains a number of roundworms called vinegar 

 eels; they can be seen only with the aid of a microscope. 



(7) The Platyhelminthes. The PLATYHELMINTHES or fiat- 

 worms are also worm-like and unsegmented. The best known 

 members are the tapeworms, which are parasitic in man and other 

 animals. The liver fluke is a serious pest; it inhabits the bile 

 ducts of sheep and causes the death of large numbers of infected 

 individuals in certain localities. ' Less widely known are the fresh- 

 water flatworms, like Planaria (Fig. 97), and the terrestrial and 

 marine forms. 



(8) The Coelenterata. The CCELENTERATA are mostly 

 marine animals, and are known as hydroids (Fig. 73) and jelly- 

 fishes (Fig. 82). Their bodies are fundamentally simple sacs, 

 although many modifications give the impression of great com- 

 plexity. Some ccelenterates are famous for the rigid skeletal 

 structures they produce; this is true of the coral polyps (Fig. 86), 

 which have even built up entire islands. There are only a few 

 fresh- water ccelenterates; one of these, Hydra (Fig. 65), is com- 

 paratively common, and is studied as a type of this group by 

 most students of biology. 



(9) The Porifera. The PORIFERA are sponges. The ordinary 

 bath sponge is the horny skeleton of an animal that lives in the 

 sea (Fig. 63). Venus' s flower basket (Fig. 62) is a sponge skele- 

 ton that is often seen in museums. Most of the sponges secrete 

 a supporting framework of calcium carbonate or silica. Only a 

 few of the sponges live in fresh water, and none lives on land. 



(10) The Protozoa. The PROTOZOA (Chap. II) are in most 

 cases so small as to be visible only with the microscope. They 



