ioo MANUAL OF ZOOLOGY SECT, iv 



medusae are shed into the water and carried by currents- to 

 the females, impregnating the ova, which thus become 

 oosperms or unicellular embryos. The oosperm undergoes 

 complete segmentation (Fig. 43, A F), and is converted 

 into an ovoidal ciliated body called a planula (G, H). The 

 planula swims freely for a time (H\ and then settles down 

 on a piece of timber, sea-weed, etc., fixes itself by one end 

 (K\ and becomes converted into &hydrula or simple polype 

 (Z, M\ having a disc of attachment at its proximal end, 

 and at its distal end a manubrium and circlet of tentacles. 

 Soon the hydrula sends out lateral buds, and, by a frequent 

 repetition of this process, becomes converted into the 

 complex Obelia-colony with which we started. 



This remarkable life-history furnishes the first example we 

 have yet met with of alternation of generations^ or metagenesis. 

 The Obelia-colony is sexless, having no gonads, and 

 developing only by the asexual process of budding ; but 

 certain of its buds the medusae develop gonads, and from 

 their impregnated eggs new Obelia-colonies arise. We thus 

 have an alternation of an asexual generation the Obelia- 

 colony, with a sexual generation the medusa. 



The majority of the Hydrozoa resemble Obelia in 

 forming fixed colonies ; but there are a few exceptional 

 cases in which the animal remains simple. One of these is 

 Hydra, the Fresh-water Polype. In Hydra the entire 

 organism (Fig. 44) consists of a simple cylindrical body with 

 a conical hypostome and a circlet of from six to eight tentacles. 

 It is ordinarily attached, by virtue of a sticky secretion 

 from the proximal end, to weeds, etc., but is capable of 

 detaching itself and moving from place to place after the 

 manner of a looping caterpillar. The tentacles are hollow, 

 and communicate freely with the enteron. There is no 

 perisarc. Buds are produced which develop into Hydras ; 



