HOMOLOGIES. 



195 



Fig. GS. 



chambers («)— the cavities of the tentacles ; while such portions of tlie tentacles of the ITjjdra 

 us still continue free will be represented by a single circle of the tentacles of the Actinia (woodcut, 

 fig. 02, a). 



Having thus established a fundamental identity between the regions of an Actinia and of 

 a Hydra, there will be no difficulty in recognising the relations between an Actinia and a 

 hydroid medusa (woodcuts, figs GG and G7) ; for as we have attempted to prove in a former 

 part of this Monograph (sec p. 40) the tentacles of a Hydra are represented by the radiating 

 canals and those extensions of them which form the primary marginal tentacles of the medusa. 



The distal ends of the radiating lamellae in Actinia are perforated each by an opening 

 through which the radiating chambers communicate with one another (woodcut, fig. 62, c). 

 Agassiz has compared these openings to the circular canal of a medusa, and I believe that in this 

 view he has correctly expressed the relations in question. 



If we further add that the generative apparatus (woodcut, fig. C2, d) is borne by the 

 radiating partitions, we shall have all the leading points in the morphology of an Actinozoon. 



Acomparison of thevarious orders of the HTDROZOAwith one another will result in the detection 

 of close homological cori'espondencies, and will throw important light on the morphology of each. 



Between a siphonophore and a hydroid the homology is so obvious as to be instantly 

 recognisable. The siphonophore (woodcut, fig. 68) as well as the hydroid presents us with a 

 colony of zooids kept in organic union with one another by 

 means of a common connecting basis or ca?nosarc ; but this 

 ccenosarc, instead of being fixed, as in the IIydkoida, is in the 

 SiPHONOPHORA invariably free and provided with a special 

 apparatus for natation. 



In consequence of the great extent to which heteromor- 

 phism is carried among the zooids composing a siphonophoral 

 colony, we can scarcely institute a satisfactory comparison between 

 the two orders without determining the homologies of each kind 

 of zooid in the siphonophore. Beginning with the polypites or 

 alimentaiy zooids (e) of the siphonophore, and comparing these 

 with the hydranths of a hydroid, we shall find the two forms to 

 agree in almost every point, except in the number and position 

 of the tentacles, which in the siphonophore are reduced to a single 

 one (/), springing, in all the typical siphonophores, from the base 

 or proximal end of the polypite. The branched condition of the 

 tentacle in the siphonophore is in no respect inconsistent with 

 this comparison ; and even if it were necessary to find a parallel 

 to it among the IItdroida, wc should have this in the branching 

 tentacles of Cladoconjne. 



The hydrocysts {g) of the siphonophore are plainly 

 arrested polypites in which the mouth has never become deve- 

 loped. 



Again, the generative zooids (/) are exactly paralleled by 

 those of the Htdroida, and are, like them, referable to two 

 types, expressed in the Hydroida by the phanerocodouic and 



Diagram of a Siphonophore. 



e, Polypite;/, tentacle of polypite; 

 /', bruuches given off by the tentacle ; 

 y, liydrocyst ; h, tentacle of hydrocyst ; 

 i, frenerative zooid representing tlie 

 jiliiinerocodonic gonophore of a hydroid ; 

 k, k, nectocalices ; I, bract ; m, m, cceno- 

 sarc ; n, pneuuiatocyst. 



26 



