IV 



PHYLUM CCELENTERATA 



229 



as the case may be leads ; the absence of a well-developed meso- 

 derm, the fixed mode of life, and associated with it, the tendency 

 to form compound structures or colonies by a process of budding. 

 In addition, the occurrence of larval stages which have at least 

 a superficial correspondence in the two phyla would appear to 

 constitute an important connecting link. But a closer examina- 

 tion of the subject shows that some of these apparent points of 

 resemblance are superficial only, and establishes a number of 

 differences between Sponges and Coelenterates too important to 

 allow us to suppose that a close relationship exists. One of these 

 differences stands out beyond the others as the most radical. The 

 osculum of a sponge is found, when we trace the development of 



Hexactir\ia v Madreporaria 

 Cestida 



Lobata / Rhizostomeae SemostomOfe 



Cydippidav / / Beroida 



Coronatcfc 



Cub x omeduscfc 



ACTINULA 

 Lucernarida- 



onaria 



Hydrocorallinde 

 Leptolincfe 



SCYPHULA 



Siponophora 

 Trachylindfe 



HYDRULA 

 FIG. 174. Diagram illustrating the mutual relationships of the Coelenterata. 



the larva, to correspond in no sense with the mouth of the 

 Coelenterate. This alone, apart from important differences in 

 the adult structure, such as the presence in the wall of the Sponge 

 of the system of inhalant apertures, the presence of the peculiar 

 collared endoderm cells, and the absence of stinging capsules, 

 would suffice to remove the Sponges from the Ccelenterata, and 

 place them in a phylum apart. But not only is the grouping 

 of Sponges and Coelenterates in one phylum thus rendered 

 impossible by important differences in their structure and develop- 

 ment ; a comparison of the mode of formation of the embryonic 

 layers in the two groups shows such radical dissimilarity that it is 

 scarcely possible to find sufficient evidence for regarding them as 

 having been derived from the same metazoan ancestors, and there 

 is much to be said in favour of the view that they have originated 

 separately from the Protozoa. 



