52 COMPARATIVE ANATOMY 



thelium. It has further been shown (p. i6) that in the redia of 

 the liver-fluke there is a body-cavity surrounding the gut, and 

 that in this case also reproductive cells are formed from the 

 epithelium lining the cavity, which obviously functions as a 

 generative sac or pouch. Putting all these facts together, the 

 invariable association between body-cavity and reproductive 

 cells suggests the explanation that the body-cavity was originally 

 nothing more than a reproductive pouch, just as it is in the 

 redia, and the more complicated arrangements which we see in 

 the Annelids are secondary modifications acquired in the course 

 of evolution in correspondence with the growing needs of the 

 organisms in which they occur. 



Adopting this as the most probable explanation of the origin 

 of thecoelom, we may further infer that metameric segmentation 

 owes its origin to a repetitive multiplication of generative 

 pouches. Instances of an analogous multiplication of the 

 generative organs may be found in the Platyhelmia. In the 

 Turbellaria the reproductive organs are multiplied, and in some 

 cases exhibit a paired arrangement. In the Cestoda they are 

 commonly repeated and arranged in series, and in the more 

 elaborate forms each proglottis is a detachable reproductive 

 segment provided with a complete set of gonads and accessory 

 glands with their ducts. The repetition of the reproductive 

 organs, admitting of a vast increase in the fertility of an organ- 

 ism, is clearly of advantage in the struggle for existence, for 

 the more numerous the progeny the greater the chance of 

 the survival of the species. In free-living, non-parasitic animals 

 like chstopod worms it is evident that the conditions of 

 existence would necessitate the development of the muscular, 

 alimentary, nervous, and excretory systems part passu with the 

 multiplication of the generative organs. Integration would go 

 hand-in-hand with repetition of parts. 



This explanation of metameric segmentation is, of course, 

 extremely hypothetical, but it accords better than any other 

 with known embryological facts. The reader should be careful 

 to distinguish the facts from the hypothesis. The former, in so 

 far as they are records of truthful observation, will ever be a 

 valuable part of our scientific assets, but the discovery of new 

 facts may make the old hypothesis untenable, and give rise to 

 new interpretations of the old facts. 



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