^^ THE NEW EVOLUTION 



z. The flukes, in which the budding off of young, which are 

 different from the adult, takes place within the larval form. In 

 the flukes the first larva, a miracidium (fig. 96, p. 175), generally 

 becomes a sporocyst (fig. 98, p. 175), which is a hollow sack with 

 excretory canals in its walls containing in its interior cavity a 

 number of germ cells. The germ cells within the sporocyst usu- 

 ally develop into redia^ (fig. 97, p. 175), which resemble sporo- 

 cysts except in having a mouth and intestine, and two lateral 

 processes near the hind end. The germ cells within the redia^ 

 usually produce cercarias (fig. 99, p. 175), which are essentially 

 young flukes with a slender and very mobile tail. Later the tail 

 is lost, and the young fluke grows into the adult (fig. 55,0. 97). 



If we are to maintain that the animal world has an underlying 

 plan or system — that it is not chaotic — there must be some sig- 

 nificance in the extraordinary developmental history of the flukes. 

 To say that it is a response to, or was developed because of, their 

 parasitic habits is simply to beg the question. If parasitic crea- 

 tures are to be considered as derived from other forms of animal 

 life, then every peculiarity of a parasite must be explained by, 

 and may itself explain, other features found in other animals. 

 In other words, no parasites can add anything to their funda- 

 mental structural equipment or to their ontogeny that does not 

 exist in related types. 



What structural peculiarities in the more complex animals 

 may be explained by the development of the flukes? The very 

 extensive asexual reproduction in the young stages of the flukes 

 recalls similarly extensive asexual reproduction in the young, 

 and often also later, stages in the coelenterates. Many coelen- 

 terates as they grow produce a large colony of interconnected ani- 

 mals of more or less plant-like form. In various coelenterates 

 the individual animals in these colonies are not all alike, but are 

 divided into three types. These three types are: Firsf, the nu- 

 tritive or sack-like individuals, which do the eating for the entire 

 colony; second, the reproductive individuals, which produce the 

 eggs; and third, the "defensive" individuals, which serve to pro- 

 tect the colony by means of structures the chief feature of which 

 is a poisonous secretion. 



In the more complex animals there is developed, typically by 

 budding from the enteron — the digestive cavity primarily derived 

 from the endoderm — a structure or organ of the greatest impor- 



[2-52-] 



