COELOMATA. 315 



the enteron concerned with the renal and with the reproductive 

 functions. 



The Platyhelminthes, Nematlielminthes, and Rotifer a are also not 

 Coelomata; or perhaps it would be better to say that they cannot 

 be ranked as Coelomata in the present state of our knowledge. It 

 may be that in these phyla also, or in some of them, the progress 

 of research will show that the renal and reproductive organs are 

 enteric in origin, and are to be regarded as homologous with the 

 coelom. 



It has already (p. 312) been pointed out that in the lower Tur- 

 bellaria there is reason to believe, from a study of the adult structure, 

 that the whole of the mesodermal parenchyma, of which the genital 

 and renal organs are part, is in reality a portion of the enteric wall. 



Finally, before leaving this subject, there is one more point to 

 notice with regard to the coelom. In all metamerically segmented 

 animals the coelom is the first organ to show segmentation in the 

 embryo : it is the first organ to show that repetition along the long 

 axis which constitutes metameric segmentation. In fact, we may lay 

 it down as a general law that in metamerically segmented animals 

 the coelom is not only the first organ to present segmentation, but 

 is segmented from its very first appearance, and that the subsequent 

 segmentation or repetition of other organs is evolved out of this 

 primal segmentation. 



There are a few apparent exceptions to this law amongst certain groups in 

 which the number of segments is very small, being, in fact, limited to three. 

 These exceptions are the Brachiopoda, Chactognat/ia, and the Echinodermata. 

 In the first two of these groups the coelom is said to be separated off as a sin<ilc 

 sue on each side, which, presumably, subsequently becomes divided into the three 

 sacs characteristic of the adult (for qualification with regard to Brachiopods see 

 chapter on Brachiopoda). In the Echinoderms there is some variation with 

 regard to this point. 



