228 



ZOOLOGY 



short canals, each of which terminates blindly at the base of the 

 corresponding tentacular sheath. Only two tentacles are present, 

 with sheaths at their bases : these are situated, not on the margin 

 of the bell, as in a medusse, but between it and the apex. There 

 are no traces of swimming plates, and, so far as the evidence at 

 present forthcoming goes, there is not sufficient evidence to 

 establish Ctenophoran affinities. 



On the other hand, the resemblance between transverse sections 

 of an embryo Ctenophore (Fig. 173, B) and of an embryo Actinian 



end 



ect 



end 



Fig. 173. — Transverse section of embryos of Actinia (A) and Beroe (B), ect. ectoderm ; 

 end. endoderm ; inf. infundibulum. (After Chun.) 



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

 dseum, and of gonads developed in connection with the endoderm 

 and discharging their products through the mouth, may be taken 

 as further evidences of affinity between the Ctenophora and the 

 Actinozoa. 



The special characteristics of the Ctenophora are, however, so 

 numerous and so striking, and their development so utterly unlike 

 that of any of the other Ccelenterata, that in our present- state of 

 knowledge it is impossible to determine their affinity with the 

 other classes with any degree of certainty. 



As to the orders of Ctenophora, it seems tolerably clear that 

 both Lobata and Cestida are derived from cydippid forms, since 

 they both pass through, in the course of development, a stage 

 closely resembling the lower Cydippida. The Beroi'da are more 

 highly organised in certain respects, e.g. in the details of their 

 histology, than the other Ctenophora, and it seems quite possible 

 that they may be derived from tentaculate forms. Whether the 

 Platyctenea are primitive or specially modified, remains doubtful, 

 especially in the absence of data regarding their development ; 

 but the latter appears the more probable conclusion. 



These relationships are expressed in the diagram on the 

 opposite page. 



By many authors the Sponges have been looked upon as so 

 closely related to the Ccelenterata that they may be regarded 

 as members of the same great phylum. The points of resemblance 

 are readily to be recognised: the simple structure, with the large cen- 

 tral cavity into which a wide opening — the mouth or the osculum, 



