108 MORPHOLOGY. 



time it must be borne in mind tiiat it is "the individual" in the somewhat technical sense of the 

 component of a biological species which is to be here understood, and that individuality, in its 

 more ordinary acceptation, cannot be excluded from our conception of the life-series of a hydroid. 

 In this sense every zooid has an individuality of its own — an individuality, however, of a very 

 different kind from that which characterises the successively repeated groups of zooids constituting 

 the individuals which logically make up the biological species. 



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

 plant continues the vegetable axis ; the gonophorc, on the other hand, has no power of con- 

 tinuing 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 hydrauths 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 to the 

 distal end of the hydrosoma, so that the older buds are met with towards the base or hydro- 

 rhizal 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 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 genera, the gonophores succeed one another, sometimes {Hydractinia ecldnata) 

 from the proximal towards the distal end of the blastostyle, sometimes {Bicoryne covfertd) from 

 the distal towards the proximal. In Tuhulariu 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 Corymorpliu. 



Where no special gonosomal axis is developed the succession is usually from the proximal 

 to the distal extremity of the branch [Bouyainvillia, Periyonimus), thus corresponding to that 

 of the zooids of the trophosome. Sometimes, however {Syncoryne, Gemmnria), it is from the 

 distal to the proximal. 



We have thus, then, in the gonosome of the IIyduoida, as in the inflorescence of plants, 

 both a centripetal and a centrifugal order of succession. It is possible, however, that irregu- 

 larities may occur, and that a new bud may be abnormally emitted at the distal side of a 

 centrifu<ral series, or at the proximal 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 hydroids as possess a special gonosomal axis. In Ttihulana indivmi 

 (PI. XX, fig. 2, 3), and in the male colonies of Tuhidaria larynx (PI. XXI, fig. 1) the gonophores 

 are— like the flowers of a raceme — carried on short pedicels along the sides of a long common 

 peduncle which springs from the body of the hydranth (woodcut, fig. 44). Their order of develop- 

 ment, however, is centrifugal, or from the distal to the proximal extremity of the peduncle, so that 

 the whole group may be compared to a reversed raceme. In the female colonies of Tiibularia 

 larynx (PI. XXI, fig. 2, and woodcut, fig. 45), and in Corymorpha nutans (PI. XIX, figs. 1 and 

 3), the pedicels become branched, with a similar order of development which thus gives us the 

 compound reversed raceme or cyme. 



