THE GROUPS OF ANIMALS 



265 



The planarias, which are free-hving, Hve under stones or logs in fresh 

 water. They have remarkable powers of regeneration, and have been 

 used by many investigators to study the physiology of development and 

 growth. The theory of gradients (page 217) in embryonic development 

 originally grew out of studies on planarias. 



The flukes are parasitic. Some of them are external' parasites, as on 

 the gills of fishes or other aquatic animals. Others — and these are the 

 menacing ones — are internal parasites. Some of the latter pass through 

 very complicated life cycles, in which the successive generations are 

 totally different in form. Usually these different types of individuals 

 must live in different hosts, one of which is a snail, the others being 

 usually arthropods (Phylum 9, below) and vertebrate animals. One such 

 life cycle involves four dift'erent hosts, following one another in a certain 



Fig. 221. — Planaria. 



Cvari/ 

 tMdef 



Fig. 222. — A fluke. 

 {From Van Cleave.) 



Fig. 223.— a tape- 

 worm. 



order. Sometimes the host, of any of the several successive general 

 types, must be a particular species — a certain species of snail, a specific 

 arthropod, a definite vertebrate species; in other trematodes there is a 

 choice of species for host, but usually only a very limited one. Some 

 degeneration (loss of eyespots, reduction of sense organs and nervous 

 system) has been permitted by the parasitic mode of life, but the repro- 

 ductive system is highly developed and specialized. 



The tapeworms are parasitic in the digestive tracts of vertebrate 

 animals. They consist of chains of rectangular individuals budded off 

 from a small "head" which is attached to the intestinal wall of the host. 

 There is no digestive tract, and no use for one since all food is absorbed 

 already digested by the host. Longitudinal nerves and longitudinal 

 excretory tubes pass along the margins of the "tape," common to all the 

 individuals in it; but each individual has its own highly developed repro- 

 ductive system which makes up most of the substance of the animal. 

 Man gets his commonest tapeworms from insufficiently cooked pork; 

 thorough cooking is the best guarantee against infection. 



