224 ZOOLOGY 



SECT. 



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 the Hydra, the individual zooids 

 being ultimately set free as medusae. The next stage would be 

 the establishment of a division of labour, in virtue of which a 

 certain proportion only of the zooids became medusae, the rest 

 retaining the polype-form, remaining permanently attached, and 

 serving for the nourishment of the asexual colony. 



The Hydrocoralliua 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. 153). 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, 

 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 Semostomae, Cubomedusae, "and 

 Bhizostomae clearly represent three grades of increasing com- 

 plexity along the same general hue of descent, the Coronata 

 diverging somewhat. It is to be noted, however, that such a 

 supposed line does not lead towards the simpler Actinozoa, but 

 towards a type which diverges from the latter as well as from the 

 Lucernarida, Cubomedusae, and Peromedusae in the absence of 

 septa or mesenteries in the adult condition. 



The close similarity of Edwardsia and the Alcyonaria in the 

 number and arrangement of the mesenteries seems to indicate the 

 derivation of both Zoantharia and Alcyonaria from a common 

 ancestor in the form of a simple actinozoan polype or actinula. 

 Edwardsia clearly leads us to the Hexactiniae or typical Sea- 

 anemones, and the Madreporaria are undoubtedly to be looked 

 upon as skeleton-forming Hexactiniae. 



The relationships of the Ctenophora to the other Ccelenterata are 

 very doubtful. Ctenaria, one of the Anthomedusae (Fig. 110, 1), 

 presents some remarkable resemblances to a Cydippid, such as 

 Hormiphora. It has two tentacles, situated in opposite per-radii, 

 and each having at its base a deep pouch in the umbrella resem- 

 bling the sheath of Hormiphora. There are eight radial canals 

 formed by the bifurcation of four inter-radial offshoots of the 



