THE CTENOPHORA 15 



Medusae which are not endowed with superficial Ctenophore-like 

 characters. The so-called infundibulum of Ctenaria proves to be a 

 brood pouch similar to that in the allied Eleutheria, and the Medusa 

 is devoid of any trace of the aboral sense organ so characteristic 

 of the Ctenophore. The position of the gonads is also different 

 in the two forms. The gelatinous tissue and the musculature of 

 the Ctenophora are mesoblastic, in the Anthomedusan they are 

 ectodermal in origin. Add to this the fact that the locomotion of 

 the Ctenophora is essentially ciliary, that of the Medusae muscular, 

 that the symmetry of the one group is radial, whilst in the other 

 it is biradial, and it must be conceded that the Medusoid affinities 

 of the Ctenophora are untenable. 



A comparison of the Ctenophora with the Anthozoa offers more 

 satisfactory grounds of homology. The ciliated ectoderm of the 

 Anthozoa might possibly be the antecedent of the specialised 

 ciliated bands which form the costae of the Ctenophora. The 

 stomodaeum of the Ctenophora and Anthozoa may fairly be 

 homologised. In both cases it is compressed in a plane which is 

 known as the sagittal plane, and in both cases the gastrovascular 

 system exhibits a biradial symmetry with regard to that plane. 

 Further evidence is afforded by the comparison of developmental 

 stages. In both the Anthozoa and the Ctenophora there is a stage in 

 which the gut is produced into four saccular pouches, so that the 

 embryo has a four-rayed symmetry. This condition, which is 

 typical in the Ctenophora, is best seen in the young Arachnactis 

 amongst the Anthozoa, but may also be distinguished in the larvae 

 of Actinidae. It would be idle to deny the significance of these 

 features, but it must be recollected that the Ctenophora have many 

 features peculiar to themselves. The costae and their combs, though 

 doubtless a specialisation of a primitively uniformly ciliated surface, 

 are characteristic of Ctenophora; so is the aboral sense organ, 

 to which there is no parallel in Anthozoa. The solid muscular 

 tentacles of the Ctenophores cannot be homologised with the 

 hollow tentacles of the Anthozoan. There is no epithelio-muscular 

 system in Ctenophora, and the musculature differs both in origin 

 and in structure from that of Anthozoa, and indeed all other 

 Coelentera. The nematocysts so characteristic of Coelentera are 

 replaced in Ctenophora by the lasso-cells, structures of an entirely 

 different nature. 



Finally, there are those who would question whether any 

 animals possessing a mesoblast can properly be called Coelentera. 

 The Coelentera, as originally denned by Leuckart, are animals 

 in which there is no body cavity or coelom separate from the 

 digestive cavity or enteron ; the two being represented by a 

 single cavity, the gastrovascular cavity or coelenteron. According 

 to this definition the Ctenophora are certainly Coelentera. In 



