PHYLUM C(ELENTERAT\ 213 



THE RELATIONSHIPS OF THE CCELENTERATA 



There can be little doubt that the lowest coelenterate form 

 known to us is the simple hydrozoan polype, represented by 

 Hydra and by the hydrula stage of many Hydrozoa. Somewhat 

 more complex, in virtue of its stomodaeum and gastric ridges and 

 filaments, is the scyphozoan polype, represented by the Scyphula 

 of Aurelia. Still more complex is the actinozoan polype, or 

 Actinula, as it may be called, with its large stomodreum, mesen- 

 teries and mesenteric filaments, and elaborate muscular system. 

 Speaking generally, one may say that these three polype-forms 

 represent as many grades of organisation along a single line of 

 descent. 



The medusa-form in the Hydrozoa is, as we have seen, readily 

 derived from the hydrula by the widening out of the tentacular 

 region into an umbrella. We may thus conceive of the Trachy- 

 lime, or hydroid medusae with no fixed zoophyte stage, as being 

 derived from a pelagic hydrula. 



The Leptolinse may be considered to have arisen in consequence 

 of the adoption of asexual multiplication, by budding, during the 

 larval or hydrula stage. Instead of the hydrula giving rise 

 directly to a medusa, we may suppose it to have formed a temporary 

 colony, by budding, after the manner of Hydra, the individual 

 zooids being ultimately set free as medusa?. The next stage 

 would be the establishment of a division of labour, in virtue 

 of which a certain proportion only of the zooids became medusa?, 

 the rest retaining the polype-form, remaining permanently 

 attached, and serving for the nourishment of the asexual colony. 



The Hydrocorallina appear to be a special development of the 

 leptoline stock, the nearest affinities of the order being with such 

 forms as Hydractinia. 



The Siphonophora may be conceived as having originated from 

 a hydrula specially modified for pelagic life by the conversion of 

 the basal disc into a float something after the fashion of Minyas 

 (Fig. 139). In such a form extensive budding, accompanied by 

 division of labour, would give rise to the complex siphonophoran 

 colony. 



The lowest Scyphozoa are the Stauromedusa 1 , some of which, 

 however, show evidence of degeneration, so that it is quite possible 

 to conceive them as having been derived from more highly 

 organised forms, instead of springing directly from simple polypes 

 of the Scyphula type. The Cannostonue, Semostoma^,. and 

 Rhizostomae clearly represent three grades of increasing com- 

 plexity along the same general line of descent. So little is known 



