214 ZOOLOGY SECT. 



of the development of the remaining Scyphozoa that it is advisable 

 to leave their position an open question. 



The close similarity of Edwardsia and the Alcyonaria in the 

 number and arrangement of the mesenteries seems to indicate the 

 derivation of both Zoantharia and Alcyonaria from a common 

 ancestor in the form of a simple actinozoan polype or actinula. 

 Edwardsia clearly leads us to the Hexactinia^ or typical Sea- 

 anemones, and the Madreporaria are undoubtedly to be looked 

 upon as skeleton- forming Hexactinia?. 



The relationships of the Ctenophora to the other Coelenterata are 

 very doubtful Ctenaria, one of the Anthomedusa3(Fig. 96, ^presents 

 some remarkable resemblances to a Cydippid, such as Hormiphora. 

 It has two tentacles, situated in opposite per-radii, and each having 

 at its base a deep pouch in the umbrella resembling the sheath of 

 Hormosipa. There are eight radial canals formed by the bifur- 

 cation of four inter-radial offshoots of the stomach, and corre- 

 sponding with them are eight bands of nematocysts diverging 

 from the apex of the ex-umbrella. If these striking resemblance^ 

 indicate true homologies we must compare the whole sub-umbrellar 

 cavity of Ctenaria with the stomodseum of Hormiphora, the 

 margin of the bell of Ctenaria with the mouth of Hormiphora, 

 and the mouth of Ctenaria with the aperture between the 

 stomodaaum and the infundibulum of Hormiphora. But, as wo 

 have seen, the gullet of Ctenophora is a true stomodseum de- 

 veloped as an in-pushing of the oral ectoderm, and has there- 

 fore a totally different origin from the sub-umbrella of a 

 medusa. Moreover the tentacles of Ctenaria have no muscular 

 base contained in the sheath, but spring from the margin of 

 the umbrella as in other Hyclrozoa : its gonads are developed in 

 the manubrium, not in the radial canals, and there is no trace of 

 an aboral sense-organ. 



On the other hand, the resemblance between transverse sections 



end 



Fi .. J CO. Transverse section <>f embryos of Actinia (A) and Beroe (13), u-t. ectoderm ; 

 oi''. endoderm ; inf. iufuiidibulum. (After Chun.) 



of an embryo Ctenophore (Fig. 160, B) and of an embryo Actinian 

 (A) is very striking, and the presence of a well-developed stomo- 

 , and of gonads developed in connection with the endoderm 



