90 MORPHOLOGY. 



though short and thick, will have thus soon attained the full number which we meet with in the 

 adult. They consist in this stage of an endodermal and an ectodermal layer, the ectoderm apparently 

 formed of a single layer of prismatic cells, while the endoderm seems to fill the entire axis with a 

 mass of minute, spherical, loosely aggregated cells. Just behind the tentacles the body of tiie 

 young hydranth is seen to be excavated by a large cavity, in which is a multitude of loose spherical 

 cells, filled with a red granular pigment, and undoubtedly thrown off from the inner surface of 

 the walls. 



The whole of the young hydroid is still completely enveloped by the delicate chitinous 

 perisarc, which forms a sheath extending over even the distal free extremity, and within which 

 the various changes just described, including even the formation of the tentacles, have been going 

 on. We now find, however, that this sheath (which has for some time lain loosely over the 

 distal parts of the hydroid, and which it seemed to invest as in a sac) becomes ruptured in front 

 of the tentacles, so that the water gains direct access to the surface of the young hydranth, and the 

 tentacles have full freedom to extend themselves. It would seem, too, that the distal extremity 

 of the proboscis had now, for the first time, become perforated by a mouth ; for up to this stage, 

 no undoubted evidence of an oral aperture could be detected. 



The young Eiidendrium has thus acquired the form of a true hydranth borne on the extremity 

 of a short simple cylindrical stem, which still springs from the centre of the radiating disc (fig. IG). 

 The ste7u elongates itself, and the body, tentacles, and hypostomc rapidly acquire all the characters 

 of the adult. It still, however, remains for it to develop from its base a creeping stolon which 

 will take the place of the primordial disc, and which would seem to originate in the elongation of 

 some of the lobes of this disc, to complicate itself by the budding of new hydranths and the 

 development of branches, and, finally, by the formation of sexual zooids, to combine a gonosome 

 with a trophosome, in order that the little hydroid whose progressive changes we have been 

 thus following may attain the condition of the adult Eudendrium (figs. I and 2). 



BeveJopnent hy Actinidce. — The developmental phenomena above described are, in all their 

 essential points, so far as we know, universal among the Hydroid a, with the exception of the 

 genera Tuhdaria, Hydra, and, probalily, also Myriotliela and Adinogonium. 



In Tuhdaria a minutely granidar plasma, which, except in its more obviously cellular struc- 

 ture, is entirely similar to that which in other Ilydroida becomes differentiated into ordinary ova, 

 may be seen enveloping the spadix of the young gonophore. Instead, however, of becoming 

 transformed in the usual way into ova, portions become detached from the mass and lie loose in 

 the cavity of the gonophore, where they undergo a development into free embryos in the manner 

 to be presently described, while the residual plasma continues to detach from its mass fresh 

 fragments, which are in their turn transformed into embryos (PI. XX, fig. 3, and PI. XXIII, 

 figs. 11, 13, 14, 15, 16, 23, 24). 



In the portions (PL XXIII, figs. \\g and 23 y) thus successively detached from the central 

 plasma (/) it is impossible to detect any decided trace of germinal vesicle or germinal spot, and yet 

 we should certainly not be justified in regarding tliem as mere gemmae, or in attributing to them 

 any other significance than that of true ova,' though, possibly, of ova after they had already passed 



' Agassiz calls the central plasma in Tuhdaria the " gerra-basis," and refuses to regard as ova 

 the masses which are thrown off from it and become developed into hydriform young. {Op. cit., vol iv, 

 pp. 255 and 2G9.) 



