THE DEVELOPMENT OF THK IIYDROZOA. 133 



directly into the adult condition. But, in most Discoph 

 the embryo becomes a fixed actinula (the so-called //</>//// 

 tuba or Scyphistoma, Fig. 28, 1.), multiplies a^aiuM-' M- ueally 

 by budding, and gives rise to permanent e< >I<>uies of Ilvdii- 

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

 enlarge and undergo a further airanuMreiiei ic mnltiplieai inn 

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

 into a number of eight-iobed discoidal medusoids (" Ephyroe" 

 or " Medusce bifida?." Fig. 28, H. and III.), and thus passes 

 into what has been termed the ^tmbila stage. The Ephyrw, 

 becoming detached from one another and from the stalk of 

 the Nrnhihi, are set free, and, underlining a great increase 

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

 reproductive organs. The base of the ^fi-<>l>il<i may develop 

 tentacles (Fig. 28, II.) and resume the Scyphittoma condition. 



Metschnikoif J has recently traced out the development of 

 Geryonia (Car marina), Polyxenia,*3ffinop9i* t and other Dis- 

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

 and in these, as in the Trachynenn/ <-l/ittun, observed by 

 Gegenbaur, 3 the process appears to be of essentially the same 

 nature as in Pekujia. The Scyphistoma of Aurelia-, Cyancea^ 

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

 Pelayiii) as a Discophore with a rudimentary disk ; in which 

 case the reproduction of the Ephyr<A-teTms of young Disco- 

 phora will not be comparable to the development of medusoid 

 gonophores among the Hydrophora, but will merely be a pro- 

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

 undeveloped, Discophore. 



In the Siphonophora? 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 which a hydrophyllium, 

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

 them, will be developed, take their origin antecedent ly to the 

 formation of the first polypite and of the gastric cavil v. 



As Metschnikoff well remarks, the mode of development 

 of the Sipkonophora is wholly inconsistent with the doctrine 

 that the various appendages of the hydrosoma in these ani- 



!"Stu<lii'n fiber die Entwiekrluni: iler Mi-.luscn und Siphonophoren." 



itxslt r(t't fur irix*. /<><>!., xxiv.) 



3 "Zur Lt'hrc ik-r (u'lu'rationswechsel." 18:>l. 



3 See especially the late observations of Metschnikoff, loc. tit. 



