460 PROFESSOR ALLMAN ON THE RELATIONS OF THE CCELENTERATA. 



Hydra as still continue free will be represented by a single circle of the ten- 

 tacles a' of Actinia. 



Fig. 1. 



Fig. 2. 



Fig. 1. — Diagramatic longitudinal section of Actinia, a, Eadiating interseptal space ; a', tentacle ; b, differentiated 

 stomach-sac ; V , somatic cavity : c, aperture in radiating septa ; d, genitalia borne by radiating septa. 



Fig. 2. — Diagramatic transverse section of Actinia, a, a, Interseptal spaces ; b, differentiated stomach-sar. 



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 aw Actinia and a hydroid medusa ; for, as I have elsewhere* attempted 

 to prove, the tentacles of a Hydra are represented by the radiating canals (figs. 



Fig. 3. 



Fig. i. 



© 



© 



Fig. 3. — Diagramatic longitudinal section of Hydra, a, Tentacle; b, hypostome ; V, somatic cavity. 



Fig. 4. — Diagramatic transverse section of Hydra through hypostome and tentacles, a, Tentacle ; b, hypostome. 



5, 6, a), and those extensions of them (fig. 5, a) which form the primary mar- 

 ginal tentacles of the medusa. 



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

 an opening (fig. 1, c), through which the radiating chambers communicate 

 with one another. Agassiz has compared these openings to the circular canal 



Report on the Reproductive System of the Hydroida. Brit. Assoc. Report for 1863. 



