34 MORPHOLOGY. 



fact that the tentacles are entirely suppressed, their place being taken by small clusters of thread- 

 cells, and that the mouth, if not wholly obliterated, is reduced to a very minute perforation, 

 which probably never subserves the function of ingestion. Near the distal extremity of these 

 bodies the gonophores (c) are borne as a dense cluster of buds. 



In the whole of the Campanularian and Sertularian hydroids the biastostyle, with its buds, 

 is enclosed in an external chitinous capsule, the gonangium (woodcut, fig. 2,/), which is never 

 present in the Tubularince. The gonangium is of very definite form for each species, and affords 

 good characters for diagnosis. 



Though it is necessary to distinguish the blastostyles from the hydranths, it cannot be over- 

 looked that they may pass into them by certain transitions. Agassiz^ describes a mouth in the 

 blastostyles of the Hgdractinia polycUna of the North American coast, but as the tentacles are 

 entirely suppressed, it is doubtful whether the orifice which here exists can be regarded as 

 destined for the ingestion of nutriment. In certain Eudendria the hydranths which carry the 

 gonophores grouped round their base present a perfectly developed form while the gonophores are 

 young; but as these continue to grow, the hydranths which carry them frequently become 

 atrophied, losing their tentacles and mouth ; and by the time the gonophores have attained to 

 maturity the hydranths have assumed the condition of blastostyles. Again, among the Sertu- 

 larince. we find in Halecium kalecinum (woodcut, fig. 29) the female biastostyle developing from 

 its summit a pair of perfect hydranths with tentacles and mouth, and with their digestive cavity in 

 communication wuth that of the biastostyle; but I know of no more instructive demonstration of 

 the relation between biastostyle and hydranth than what is afforded by the female gonangium 

 and its contents in Sertidaria rosacea, S. tamarisca, and S. falax. In these hydroids 

 (woodcuts, fig. 23 — 26) the biastostyle develops from its summit a set of peculiarly formed 

 tentacles, which, after becoming invested with a perisarc, continuous with the chitinous walls 

 of the gonangium arch, over the summit of the gonangium, so as to form the walls of a special 

 chamber, which constitutes a marsupial receptacle, iu which the ova, after their discharge from 

 the gonangium proper, may undergo further development. These, however, are all exceptional 

 cases, and do not render less valid the association of the biastostyle with the gonosome rather 

 than with the trophosome, while they are impoi'tant as showing the homological identity between 

 the hydranth and the biastostyle. 



Notwithstanding the transitions which may be thus traced between the hydranth and the 

 biastostyle, we must carefully avoid the confounding of a true biastostyle, whose characteristic 

 form and suppression of nutritive function show themselves before the appearance of the generative 

 buds, and those pseudo-blastostyles which are caused by the exhaustive action of the generative 

 buds on an ordinary hydranth. 



developed form. Such complete differentiation, however, is not always attained even in the Hydroida, 

 while among the Siphonophora a hydrozoal group possessing the closest relations with the Hydroida, 

 the margin of the gonocalyx or umbrella of tlie medusiform gonophore in the Cahjcophoridit carries 

 neither tentacles, ocelli, nor lithocysts, and the manubrium develops, at least usually, no mouth upon 

 its extremity. 



^ Agassiz, ' Contrib. to the Nat. Hist, of the United States,' vol. iv, p. 230. 



