ZOOGENESIS ^ 



related to the nematodes from which, however, they differ in 

 practically every detail of their structure. 



All of the tapeworms, spiny-headed worms, flukes and gordian 

 worms are parasites, as are also many of the thread-worms and a 

 few of the turbellarians. The tapeworms, spiny-headed worms, 

 some of the nematodes, some of the turbellarians, and a few curi- 

 ous forms (such as Rhopalura [figs. 12.7, 12.8, p. 185], Dkj/ema, 

 Dicyemo-psis [fig. 12.6, p. 185], Dicyemella^ and others) which are 

 supposed to be allied to the trematodes or flukes have no mouth 

 and no digestive system. A few of the small turbellarians with- 

 out a digestive system are said to be able to live on inorganic 

 material like a green plant through the medium of the green cells 

 in their tissues. 



In the young stages of many of these creatures, especially in 

 certain tapeworms and flukes and in a few turbellarians, asexual 

 reproduction is carried to an extraordinary degree, recalling the 

 conditions in the younger stages of many of the radially sym- 

 metrical coelenterates. Other forms, especially among the tur- 

 bellarians, possess the most remarkable powers of reparation and 

 of regeneration, being able to reproduce an entire individual from 

 a very small fragment. This also recalls the conditions in certain 

 coelenterates. On the other hand, in other forms there is simple 

 sexual reproduction only, and the individuals possess little or no 

 power of reparation. Such forms in these respects most nearly 

 resemble the more complex forms of bilaterally symmetrical 

 animals. 



In some of these creatures both sexes occur in the same individ- 

 ual, while in others the sexes are separate. In a few, unfertilized 

 eggs capable of developing are produced. Rarely, larval as well 

 as adult forms produce eggs. Many develop directly from the 

 tgg to the adult, but others have one, two, three, or even four 

 very distinct larval forms, some of which produce great numbers 

 of the same, or different, larval forms through various types of 

 asexual reproduction. Some of the larval forms are branched and 

 plant-like, and in many cases the adults are practically colonial 

 animals. 



The number of different kinds of these animals is very great, 

 and the part they play in the economy of nature, especially in pre- 

 venting the too rapid increase of other kinds of animals, is most 

 important. 



[2-43] 



