106 



COMPARATIVE ANATOMY 



CHAP. 



the stock as free-swimming Medusce. Degeneration can go so far that 

 the original Medusa organisation becomes quite unrecognisable (Fig. 

 80, C). 



The division of labour among the persons of a stock goes still 

 further in some Hydroids, and leads to the formation of polymorphic 

 stocks. Besides the ordinary nutritive and sexual persons, feelers 

 (tasters) devoid of mouth and tentacles, and thorn-like protective per- 

 sons (guard polyps) provided with a hard periderm skeleton, may 

 occur; between these latter the other persons can withdraw. 



Reproduction by gemmation and fission is relatively rare in the 

 Medusce of the Hydrozoa, and in the Craspedote Medusce generally. In 

 the division of the Anthomedusce gemmation has till now been observed 

 only in the family of the Sarsiadce. Here numerous buds are formed 

 either at the edge of the umbrella, or on the very much lengthened 



FIG. 80. A, B, C, Three different types of gonophores from Hydrozoa. e, Ectoderm ; en, 

 endoderm ; es, outer ectodermal envelope of the gonophore ; u, umbrella ; ra, radial canal ; r, circum- 

 ferential canal ; t, tentacles ; m, gastric peduncle ; o, mouth ; ov, gonade (ovary) ; v, velum. 



gastric peduncle. These buds grow into young Sarsice like the mother 

 animal, then detach themselves and swim about independently. Here 

 we have formation of free-swimming Medusa-stocks without division 

 of labour and without polymorphism of the individuals. The Medusce 

 attain full development only after their detachment from the mother 

 animal. 



Eeproduction by repeated binary fission has also been observed in 

 Craspedote Medusce. In some cases (Gastroblasta) peculiar free-swimming 

 Medusa -stocks arise which have the following structure. A single 

 Medusa provided with tentacles and marginal vesicles carries on its 

 subumbrella numerous gastric pouches. The number of these gastric 

 pouches determines the number of persons in the stock, which are 

 so far incompletely divided from each other that their discs never 

 separate. 



Asexual reproduction by a sort of fission occurs also in young 

 forms of the Discomedusce (e.g. Aurelia), i.e. in the young attached stage 

 known as Scyphistoma and described above. In the simplest case 



