( 459 ) 



XVII. — On the Homological Relations of the Ccelenterata. 

 By Professor Allman. 



(Read 29th May 1871.) 



Independently of the general agreement which necessitates the association of 

 the Hydra, Actinia, and other Coelenterate animals into one primary group of the 

 animal kingdom, we must also expect a special morphological correspondence 

 between the various forms of animals thus associated. In other words, a homo- 

 logical agreement ought to be determinable between the parts of animals included 

 in any one subordinate section of the Ccelenterata with the parts of animals 

 included in any other. 



A comparison of the two primary sections of the Ccelenterata (Actinozoa 

 and Hydrozod), and of the various orders of these with one another, will show 

 that such an agreement really exists, and that it is possible, by easily under- 

 stood and thoroughly consistent modifications, to convert each special type into 

 any of the others. 



With the view of rendering apparent these relations, we shall compare an 

 actinozoon (Actinia) with a hydrozoon (Hydra), and shall further compare with 

 one another the various orders of the Hydrozoa. 



Agassiz has compared the radiating chambers, which in an actinozoon inter- 

 vene between the stomach sac and the outer walls, with the radiating canals of a 

 medusa."" I believe that he has thus struck upon the true homologies of those 

 parts ; but when he maintains further that the differentiated stomach of an 

 actinozoon is only the proboscis (hypostome) of a hydrozoon inverted into its 

 body cavity, he suggests a conception of actinozoal homology which is incon- 

 sistent with the actual structure. 



In order to form a correct notion of the homological relations between an 

 Actinia and a Hydra, we have to imagine the tentacles of & Hydra (figs. 3, 4) for 

 a greater or less extent connate with the sides of the hypostome and with one 

 another. The hypostome of the Hydra, while retaining its normal position, will 

 thus become the stomach of the Actinia (figs. 1, 2, b), and this will at the same 

 time become connected with the outer walls by a series of radiating lamellae — 

 the connate tentacle-walls, — separated from one another by radiating chambers 

 a — the cavities of the tentacles, — while such portions of the tentacles of the 



* Contr. Nat. Hist. U.S. vol. iv. p. 377. 

 VOL. XXVI. PART. II. 6 O 



