PROFESSOR ALLMAN ON THE RELATIONS OF THE CCELENTERATA. 401 



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 is borne by the radiating 



Fig. 5. 



Fig. 6. 



Fig. 5. — Diagramatio longitudinal section of a Hydroid Medusa, a, Radiating canal ; a', marginal tentacle ; l>, 

 manubrium ; &', atrium; c, lumen of circular canal ; d, generative elements; r, atrial region of umbrella; i J , manubrial 

 region of umbrella ; v, velum. 



Fig. 6. — Diagramatic transverse section of Hydroid Medusa through the manubrial region of the umbrella, a, Radi- 

 ating canal ; b, manubrium ; d, generative elements ; r', manubrial region of umbrella. 



partitions, we shall have all the leading points in the morphology of an actino- 

 zoon. 



A comparison of the various orders of the Hydrozoa with one another will 

 result in the detection of close homological correspondencies, 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 (fig. 7), as well as the hydroid, pre- 

 sents us with a colony of zooids, kept in organic union with one another by 

 means of a common connecting basis or coenosarc ; but this ccenosarc, instead of 

 being fixed, as in the Hydroida, is in the Siphonophora invariably free, and 

 provided with a special apparatus for natation. 



In consequence of the great extent to which heteromorphism 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 poly- 

 pites or alimentary 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 siphono- 

 phore are reduced to a single one (/), springing, in all the typical Siphonophora, 

 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 Hydroida, we 

 should have this in the branching tentacles of Cladomryne. 



