226 



ZOOLOGY 



SECT. 



end 



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 is 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 Beroida 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 

 ten t ac ul a te 

 forms. Whether 

 the Platyctenea 

 are primitive or 

 specially modi- 

 fied r emai ns 

 doubtful ; but 

 the latter, in 

 view of the 

 larval develop- 

 ment of Tjalfiella, 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 

 central cavity into which a wide opening the mouth or the 

 osculum, as the case may be leads ; the absence of a well-developed 

 mesoderm, the fixed mode of life, and associated with it the ten- 

 dency to form compound structures by a process of budding. In 

 addition, the occurrence of larval stages which have at least a 

 superficial correspondence in the two phyla, would appear to con- 

 stitute an important connecting link. But a closer examination 

 of the subject shows that some of these apparent points of re- 

 semblance are superficial only, and establishes a number of differences 

 between Sponges and Ccelenterates too important to allow us to 

 suppose that a close relationship exists. One of these differences 

 stands out beyond the others as the most radical. The osculum 

 of a sponge is found, when we trace the development of the larva, 

 to correspond in no sense with the mouth of the Ccelenterate. 

 This alone, apart from important differences in the adult structure, 



FIG. 176. Transverse section of embryos of Actinia (A) and 

 Beroe (B). ect. ectoderm ; end. endoderm ; inf. infundibulum. 

 (After Chun.) 



