HOMOLOGIES. 



193 



Hydrozoon inverted into its body-cavity, he suggests a view wliicli is insufTicient to explain tlie 

 actual structure, for the radiating lamellae still remain unaccounted for. 



We possess very few observations on the early stages in the development of Actinia, but it 

 seems to be generally taken for granted that the first appearance of the stomach-sac is to be 

 explained by sujjposiiig a l)ending inwards of the margin of the oral aperture, and if this be 

 admitted the conception of Agassiz will so far be borne out.' But then the septa which are found 



Fig. G4. 



Fig. 65. 



Diagramatic lon*;itudinal section of 



Hydra, 

 a. Tentacles; i, hypostome; />', so- 

 matic cavity. 



© o© 



Dia^rramatic transverse section of 

 Kijdra through hypostome and 

 tentacles, 

 a, Tentacles ; i, hypostome. 



surrounding the stomach-sac must in this case be formed as independent structiu-es between the 

 outer walls and the inverted mouth-margin, and the intervening chambers must have an entirely 

 different origin from that of the radiating canals of a medusa, and can hardly be regarded as their 

 strict homologues. 



This introversion of the mouth-margin, however, is not a necessary interpretation of the 

 successive forms presented by the embryo in its early stages, for an exactly similar appearance 



' See Cobbold ('Ann. Nat. Hist.,' 2nd series, vol. xi), and more especially Kowalevsky (Nach- 

 richten der K. Gesellschaft der Wissensch. zu Gottingen,' 1868, s. 157). Kowalevsky gives no figure, 

 and Lis communication is in such a condensed form, stating the results of his observations rather than 

 the steps by which these results are obtained, that it is not always easy to discover his exact meaning, 

 especially with regard to the mode of formation of the septa and the radiating chambers. The forma- 

 tion of the stomach-sac, liowever, by the turning in of the mouth-margin is clearly maintained. 



It may be here mentioned that Kowalevsky denies the correctness of parallelising the common 

 cavity of the Cmlenterata with the true body-cavity of other groups of the animal kingdom, and main- 

 tains that the so-called body-cavity of the Cwknterata is rather the homologue of the intestine of other 

 animals, being, according to him, a secondary formation supervening on that of the proper body-cavity 

 in which it has been formed by a process of introversion of the walls, the proper body-cavity 

 itself becoming more or less obliterated. 



A view agreeing in most respects with that of Kowalevsky is also advocated by Semper {' Reise 

 im Archipel der Philippinen,' Zweiter Theil, Erster Band, p. 131), who maintains that the endoderm o 



