THE DEVELOPMENT OF THE HYDROZOA. 133 



directly into the adult condition. But, in most Discophora^ 

 the embryo becomes a fixed actinula (the so-called Hydra 

 tuba or fSci/phistoma^ Fig. 28, 1.), multiplies agamogenetically 

 by budding, and gives rise to permanent colonies of Hydri- 

 form polyps. At certain seasons of the year, some of these 

 enlarge and undergo a further agamogenetic multiplication 

 by fission (Fig. 28, II.). In fact, each divides transversely 

 into a number of eight-lobed discoidal medusoids (" Ephyrm " 

 or '•'■ 3IeduscB bifidce,''^ Fig. 28, II. and III.), and thus passes 

 into what has been termed the btrohila stage. The EjjhyrcB^ 

 becoming detached from one another and from the stalk of 

 the Strobila^ are set free, and, undergoing a great increase 

 in size, take on the form of the adult Discophore, and acquire 

 reproductive organs. The base of the Strobila may develop 

 tentacles (Fig. 28, II.) and resume the jScyphlsto^na condition. 



Metschnikoff ^ has recently traced out the development of 

 Geryo)iia(Carynarlna), Polyxenia^JEgliiopsis^ and other Z>^5- 

 cophora^ which differ from the foregoing in possessing a velum ; 

 and in these, as in the Trachynema ciliatum, observed by 

 Gegenbaur,' the process appears to be of essentially the same 

 nature as in Pelagia. The Scyphistoma of Aurelia, Gyanoia., 

 and their allies, is probably to be regarded, like the larva of 

 Pelagia^ as a Discophore with a rudimentary disk ; in which 

 case the reproduction of the jEJpjhyra4ovms of young Disco- 

 p)liora will not be comparable to the development of medusoid 

 gonophores among the HydrophoTa^ but will merely be a pro- 

 cess of multiplication, by transverse fission, of a true, though 

 undeveloped, Discophore. 



In the Siphonopliora^ the result of yelk division is the 

 formation of a ciliated body consisting of a small-celled 

 ectoderm investing a solid mass of large blastomeres, which 

 eventually pass into the cells of the endoderm. This body 

 does not take the form of an actinula. On the contrary, it 

 appears to be the rule that buds from Avhich a hydrophjdlium, 

 a nectocalyx, a tentacle, or pneumatophore, or even all of 

 them, will be developed, take their origin antecedently to the 

 formation of the first polypite and of the gastric cavity. 



As Metschnikoff well remarks, the mode of development 

 of the Siphonoiyhora is wholly inconsistent with the doctrine 

 that the various appendages of the hydrosoma in these ani- 



1 " Studien iiber die Entwickelung der Medusen und Siplionoplioreti." 

 (ZeitscJirift fiir wiRs. Zool.^ xxiv,) 



2 " Zur Lehrc der Generationswoohsel." 1^54. 



3 See especially the lute observations of Metschnikoff, loc. cit. 



