' '>0 PHYSIOLOGY. 



I have spoken above of an exception to the all hut universal fact that the generative 

 elements originate between the ectoderm and endoderm of a body homologous with the manubrium 

 of a naked-eyed medusa. The exception referred to consists in the origination of ova in the blas- 

 tostyle, as may be seen in Sertidaria pimila and one or two other species of Sertularia. 



In Sertularia pumila a solitary gonophore of the ordinary form, and containing in the usual 

 way ova or spermatozoa, originates, as in other cases, by a bud from a blastostyle. In the female 

 colonies, however, nucleated spherical bodies, in no way distinguishable from young ova, are 

 found in the walls of the blastostyle itself, between whose ectoderm and endoderm they seem to 

 lie (woodcut, fig. 21, k). I have not succeeded in satisfactorily tracing the destination of these 

 bodies ; but I have reason to believe that the true gonophores bud forth from that part of the 

 lilastostyle in which the nucleated bodies occur, and that these, as young ova, pass from the 

 blastostyle into the budding gonophore, where they would then naturally occupy their normal 

 position between the endoderm and ectoderm of an organ representing the manubrium of a 

 medusa, destined to undergo there a further development before being discharged into the acro- 

 cyst, which, as we have already seen, exists in this species. Each gonophore, after having per- 

 formed its duty as a receptacle, in which certain intermediate stages of development take place, 

 would seem to disappear, and be succeeded by another, which in a similar way receives its young 

 ova from the blastostyle on which it buds.' 



b. Non-sewiial Reproduction. 



Gemmation. — As already saiil, non-sexual or agamic reproduction may manifest itself in the 

 Hydroida either by budding or by fission. There is scarcely any part of the external surface of a 

 hydroid which may not give origin to a bud, though the actual conditions which determine the 

 formation of this bud are entirely uidcnown. Buds capable of becoming developed into one or 

 other of the various forms of zooids, which make up the hydroid colony, may be emitted by the 

 hydranth, by the hydrocaulus, or by the hydrorhiza. In the gonosome we find that not only does 

 the blastostyle give origin to buds destined to take part in the generative functions, but that the 

 medusae themselves have the power of emitting buds from various parts of their surface. The 

 form and development of these buds have already been considered in the morphological section of 

 the present Monograph. 



As an almost universal fact the bud, from whatever part of the hydroid it is emitted, has its 

 somatic cavity in open communication with that of the budder, so that the common somatic fluid 

 passes freely from the one into the other. Cases, however, have been recorded (see above, p. 82) 

 in which certain .^ginidan medusae would seem to give origin to buds from the internal surface 

 of the manubrium. It is possible that there may be here some error of observation, and, though 



' Bodies, undoubtedly of the same nature as those here described, but without any indication 

 of a nucleus, are figured by Agassiz in an American species, wliich he regards as identical with the 

 Sertularia pumila of Europe (op. cit., pi. xxxii, fig. 9). Tliey had also been already described by 

 Lindstrora (op. cit.). 



