OF ZOOIDS IN THE HYDROIDA. 103 



tributing probably in some way as yet unknown to the generative functions of 

 the hydroid, while they have no power of continuing the succession in a direct 

 or collateral line like the proper hydranths of the trophosome. 



The hydranth normally continues the axis in the hydroid colony, just as the 

 leaf-bud in the plant continues the vegetable axis ; the gonophore, on the other 

 hand, has no power of continuing the axis, and constitutes the terminal zooid 

 in each period of the series, just as the flower-bud stops the elongation of the 

 axis in the plant. This analogy, however, must not be pushed too far, for while 

 the hydranths and gonophores are simple zooids, the leaf-buds and flower-buds 

 are complex associations of the corresponding element of individuality in the 

 plant. 



The normal order of succession of the buds in the trophosome is from the 

 proximal or fixed to the distal or free end of the hyclrosoma, so that the older 

 buds are met with towards the base or hydrorhizal end of the main stem and 

 branches, the younger ones towards the summit. In the gonosome, on the 

 other hand, the order of succession is sometimes towards the distal, sometimes 

 towards the proximal end of the axis. In the calyptoblastic genera, represented 

 by campanularian, sertularian, and allied forms, the order of succession of the 

 sporosacs or blastochemes is invariably from the distal towards the proximal 

 extremity of the blastostyle on which in these genera they are always borne. 

 When a blastostyle is present in the gymnoblastic or tubularian genera, the 

 gonophores succeed one another, sometimes from the proximal towards the 

 distal end (Hydractinia echinata), sometimes from the distal towards the 

 proximal (Dicoryne conferta). In Tubularia their succession is from the distal 

 towards the proximal end of the common peduncle, which is more or less 

 developed in the various species of this genus ; and the same order of succession 

 occurs in Corymorpha. 



Where no special gonosomal axis is developed, the succession is usually 

 from the proximal to the distal extremity of the branch {Bougainmllia, Perigoni- 

 mus), thus corresponding to that of the zooids of the trophosome. Sometimes, 

 however (Syncoryne, Gemmaria), it is from the distal to the proximal. 



We have thus, then, in the gonosome of the Hydroida, as in the inflorescence 

 of plants, both a centripetal and a centrifugal order of development. It is 

 possible, however, that irregularities may occur, and that a new bud may be 

 abnormally emitted at the distal side of a centrifugal series, or at the proxi- 

 mal side of a centripetal one, so as to disturb in individual cases the normal 

 sequence of the zooids. 



Some further points admitting of comparison with the inflorescence of plants 

 may be noticed in the gonosome of such hyclroids as possess a special gonosomal 

 axis. In Tubularia indivisa (fig. 5), and in the male colonies of Tubularia 

 larynx, the gonophores are — like the flowers of a raceme — carried on short 



