194 



HOMOLOGIES. 



would he produced hy tlie prolongation of the oral margin outwardly, accompanied by an 

 extension around it of the general body-cavity.' 



According to the latter interpretation, the so-called stomach-sac would represent the hypo- 

 stome of Hydra uninverted, and surrounded by a continuation of the body-cavity. But this distal 

 prolongation of the body-cavity is farther seen to be divided by radiating septa into chambers 

 which open freely into the simple primitive body-cavity below. The exact meaning of these 

 septa and of their intervening chambers remains for determination, and is at once suggested by 

 a comparison of Actinia with Hydra. 



In order to form a correct notion of the horaological relations between an Actinia and 

 a Hydra, Vie have to imagine tlie tentacles of a Hydra (woodcuts, figs. 04 and G5, a) for a greater 

 or less extent connate with the sides of the hypostome {Ij) and with one another. The hypostome 



Fig. 66. 



Diagramatic loiia;itudliial section of a 

 H^dro'id Medusa. 



a, Radiating canals; a\ marginal ten- 

 tacles; h, manubrium; h\ atrium; c, lumen 

 of circular canal ; d, geuf rative elements ; 

 r, atrial region of umbrella ; r', manubrial 

 region of umbrella ; v, velum. 



Fig. 67. 



Dia^jramatlc transverse section 

 of a Hydroid Medusa througll 

 the manubrial region of the 

 umbrella. 



a. Radiating canals; &, manu- 

 brium ; (?, generative elements; 

 r', manubrial region of umbrella. 



of the Hydra, while retaining its normal position, will thus become the stomach-sac of the Actinia 

 (woodcuts, figs. 62 and 63, U), and this will necessarily become connected with the outer walls by 

 a series of radiating laniellc-c— the connate tentacle-walls — separated from one another by radiating 



tlie Calcnterata is formed by an introversion of the ectoderm. He asserts that the Coslcnterata have no 

 body-cavity, but only an analogue of it in a connective tissue lying between the endoderm and 

 ectoderm, and to which he assigns tlie name of Coenenchyma. He maintains that all the canals 

 which in every direction permeate the zooids and the coenosarc, are only appendages of the digestive 

 cavity, and that, consequently, the generally received view, which represents the alimentary canal of 

 the Ccelenterata as opening into the body-cavity, must be given up. 



That the views of Kowalevsky and Semper do not hold good for all the Ccelenterata there can be 

 no do\ibt. They certainly are not applicable to the Hydroida, 



^ The account of the development of the Red Coral given by Professor Lacaze Duthiers (' Hist. 

 Nat. du Corail ') bears out this interpretation, rather than that which would sec in the stomach-sac 

 an inverted hypostome. 



