226 ZOOLOGY sect. 



Gtenoplana and Cceloplana are perhaps best looked upon as forming an 

 additional, somewhat aberrant, order of the Ctenophora, viz. — 



Order 5. — Platyctenea. 



Flattened Ctenophora of creeping habit, with a pair of retractile lateral 

 tentacles. The cost* (swimming-plates), when present, are retractile. There are 

 no meridional canals, but a system of anastomosing peripheral vessels. 



THE RELATIONSHIPS OF THE CCELENTERATA. 



There can be little doubt that the lowest ccelenterate 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 stomodseum (if a true stomodseum 

 be indeed represented) and its 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 stomodseum, mesenteries 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- 

 linse, or hydroid medusas 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 the Hydra, the individual 

 zooids being ultimately set free as medusae. The next stage 

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

 of which a certain proportion only of the zooids became medusas, 

 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 basic disc into a float — something after the fashion of Minyas 

 (Fig. 152). In such a form extensive budding, accompanied by 

 division of labour, would give rise to the complex siphonophoran 

 colony. 



The lowest Scyphozoa are the Lucernarida, some of which, 



