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COMPARATIVE ANATOMY 



Hydra multiplies itself in two ways asexually, by means of 

 buds, and sexually, by the development of fertilised ova. In 

 budding the animal gives rise to a new individual very much 

 as a plant gives rise to a new shoot. At a spot about half-way 

 between the mouth and basal disc the ectoderm becomes 

 thickened by a proliferation of its interstitial cells, and then 

 all three layers, ectoderm, mesoglcea, and endoderm, are bulged 

 outwards to form a hollow projection from the surface. The 

 hollow projection enlarges to form a conical outgrowth, at the 

 distal end of which a ring of small knobs, the rudiments of the 

 future tentacles, make their appearance. The tentacles en- 

 large, a mouth is formed in their midst, and the outgrowth has 

 assumed the shape of a small Hydra whose tissues and gastro- 

 vascular cavity are continued into those of the parent form. 

 Several such buds may be formed at one time on a single 

 Hydra, and some of them may give rise by a similar process 

 to secondary buds, so that a little composite stock or colony 

 of Hydrae is produced, all having their gastrovascular cavities 

 in communication with that of the parent. Sooner or later, 

 however, the buds drop off and lead independent existences 

 as free individual Hydrae. It should be noticed that the method 

 of reproduction by budding, whilst undoubtedly asexual, differs 

 from the asexual method of reproduction observed in Volvox 

 and described as characteristic of the majority of plants. In 

 the latter case a single cell is produced which, without previous 

 fertilisation, segments and develops into a new organism. In 

 budding, the tissues of the parent pass over into the like tissues 

 of the offspring, ectoderm into ectoderm and endoderm into 

 endoderm. 



The summer months appear to be the season of sexual re- 

 production for Hydra viridis, October and November for H. 

 fused) but specimens bearing reproductive organs are never 

 very abundant. As a rule, both male and female organs are 

 produced by the same individual ; hence Hydra is said to be 

 hermaphrodite or monoecious. But several authors have de- 

 scribed broods of Hydra which produced either exclusively 

 male or exclusively female organs, so Hydra may be said to 

 be occasionally dioscious.* 



* jj-ovos, single ; 8(s, twice ; otos, a house. 



