122 COELENTERATA. 



hydrorhiza arise several individuals, some of which are ordinary 

 polyps, with a mouth and tentacles; others are polyps lacking the 

 mouth and tentacles, and acting as defensive organs for the colony 

 (the dactylozooids) ; others again, while lacking the mouth and 

 possessing only vestiges of tentacles, have the property of budding 

 off the gonophores or sexual members of the colony these are the 

 blastostyles, and the sexual individuals they produce are imperfectly 

 developed medusae, which characterize this particular species. In 

 some species the medusa acquires a complete development, and 

 breaking away from the colony swims in the sea and distributes 

 its products far and wide. 



The phenomenon of alternation of generations depends upon the 

 fact that in many Hydromedusae and Acdlephae the fertilized ovum 

 gives rise to an organism (the polyp) which can produce buds, 

 but not ova and spermatozoa. Some of the buds become Medusae 

 or medusa-like individuals in which the sexual elements are formed. 

 This kind of alternation of generations in which a sexually repro- 

 ducing generation succeeds one or more generations of asexual forms 

 is called metagenesis. 



The ovum is generally fertilized outside the parent, but in some 

 Anthozoa the fertilization and the early stages of development take 

 place in the enteric cavity. In many forms, particularly amongst 

 the Medusae, all the individuals of the same species discharge their 

 eggs at the same time of day, but this time may alter slightly 

 according to the time of year. It is remarkable that in the most 

 nearly allied species the eggs are discharged at the most different 

 times. It has been suggested that this is an arrangement to prevent 

 the crossing of closely allied species. 



Almost all Coelenterates are marine. Hydra, Microhydra, Cordy- 

 lophora amongst polyp forms; and Limnocodium, found in the 

 Victoria Regia tank of the Royal Botanical Society, London, and 

 Limnocnida from Lake Tanganyika, amongst Medusae, are the 

 most striking of the few fresh-water members of the group. The 

 polyps and polyp colonies are for the most part attached to foreign 

 bodies and lead sedentary lives, w,hile the Medusae, Siplionophora, 

 and CtenopJwra are free-swimming pelagic organisms, striking 

 for their beauty and for the extreme fragility and transparency 

 of their tissues. Many of them are phosphorescent. The greater 

 number are littoral or pelagic in habit, but a few forms, often 

 characterized by important peculiarities of structure, are from the 

 deep sea. 



