ANIMAL EVOLUTION 



535 



and (c) defensive, the defensive mechanism 

 consisting of numerous cells containing 

 a secretion and also a coiled tubule. These 

 defensive polyps are probably to be classed 

 as excretory with a secondary adaptation 

 for defense rather than as primarily 

 defensive. 



What is the special significance of this? 

 If in the formation of buds internally 

 within the original unit these buds were 

 of these three types into which the polyps 

 of many of the ccelenterates typically are 

 divided, nutritive, reproductive and ex- 

 cretory structures of a very definite sort 

 would not only become internal, but 

 would acquire a definite internal place 

 and manner of origin. 



In the animals possessing the internal 

 structure known as a ccelome, the latter 

 is divided into three sections, (a) the 

 sack-like or perivisceral ccelome, (Jf) the 

 reproductive or gonadial ccelome, and 

 (V) the excretory or nephridial ccelome. 



While we cannot trace any definite 

 connection between the three divisions of 

 the ccelome and the three types into which 

 ccelenterate polyps are divided, yet the 

 correspondence when considered in con- 

 nection with the internal budding seen 

 in the developing flukes, as contrasted 

 with the linear budding occurring in the 

 tapeworms, is so very striking that we 

 can scarcely avoid the assumption that 

 the ccelome arose from internal budding. 



(3) The turbellarians and nematodes 

 are solitary bilateral animals, both with 

 distinct traces of radial symmetry in their 

 nervous systems, and the turbellarians also 

 with a more or less radially symmetrical 

 digestive system. Each individual within 

 these groups may be compared to a single 

 ccelenterate polyp. 



(4) Such turbellarians as Microstotnum 

 are single animals, each individual being 

 comparable to a single ccelenterate polyp. 



But they divide in such a way as to pro- 

 duce a chain of similar attached animals, 

 each of which is independent of the other 

 and not an integral part of a more or less 

 unified entity. 



THE PLATWORMS AS CONNECTING LINK 



BETWEEN RADIAL AND BILATERAL 



FORMS 



The tapeworms, the flukes, the turbel- 

 larians and Microstotnum are all flatworms, 

 and all are more or less closely related to 

 each other. They all retain to a very 

 considerable degree remnants of radial 

 symmetry, as well as other ccelenterate 

 features, while at the same time they are 

 for the most part bilaterally symmetrical. 



As they, and they alone, are structurally 

 intermediate between the radially and 

 bilaterally symmetrical animals, how can 

 we avoid the conclusion that in some way 

 or other they must form the connecting 

 link between the two? 



Prejudice always clouds our vision when 

 it comes to parasites. All of the tape- 

 worms and all of the flukes are parasites. 

 They have therefore always been regarded 

 as abhorrent creatures anomalous in 

 structure, whose bizarre life history has 

 been developed in accordance with their 

 parasitic habits. 



But nature does not produce anomalies. 

 The reason that the tapeworms and the 

 flukes are parasites is simply that their 

 curious structure renders a free existence 

 impossible for them under conditions as 

 they are today. They occur only as 

 parasites because to exist at all they must 

 be protected by the body of a larger 

 creature. 



A parasitic branch from, for instance, 

 annelid (as Protomy^ostomum) or crustacean 

 (as Sacculina) stock may become so 

 changed as to lose nearly all vestiges of its 

 true relationships. But however great 



