192 



HOMOLOGIES. 



HOMOLOGICAL RELATIONS OF THE CCELENTERATA. 



Independently of the general agreement which necessitates the association of the Hijdra, 

 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 homological 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. 



Fig. G2. 



Diagramatic longitudinal s ection ofActinra. 



a. Radiating inter=eptal space; a', ten- 

 tacle; h, diflerentiated stomach-sac; 6', 

 somatic cavity; c, aperture in distal end of 

 radiating lamella; d, genetalia borne by 

 radiating lamellte. 



In this figure, as well as in figs. 64, 66, 

 69, 71, and 73, a bristle is represented as 

 having been passed from the main canity 

 of the body into a gastrovascular canal or 

 its homologue. 



Fig. 63. 



Biagramntic transverse section 

 of Actinia. 



a, o, Interseptal spaces; I, 

 differentiated stomacli-sac. 



A comparison of the two primary sections of the Ccelenterata {Actinozoa and Hyurozoa), 

 and of the various orders of tliese with one another, will show that such an agreement really 

 exists, and that it is possible, by easily luiderstood 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, woodcuts, figs. 02 and 63) with a Hydrozoon {Hydra, woodcuts, figs. 64 and 05), and 

 shall further compare with one another the various orders of the Hydrozoa. 



Agassiz was, I believe, the first to compare the radiating chambers which, in an Actinozoon, 

 intervene between the stomach-sac and the outer walls, with the radiating canals of a medusa.' 

 He seems to have thus struck upon the true homologies of these parts ; but when he maintains 

 further that the differentiated stomach of an Actinozoon is only the proboscis (hypostome) of a 



Contr. Nat. Hist. U.S.,' vol. iv, p. 377. 



