IV 



PHYLUM CCELENTERATA 



215 



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 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 tentaculate forms. 



These relationships are expressed in the following diagram : 



Madrefjoraria 



Hexachnia / 



\ / Alcy Onaria 



Cesh'da 



B e ro'i da 



EcLuiarcLsra. \ 

 Rhizost"omce \ ' 



ACTINULA 



Cannosf-omCB 



Shau.ro medusae Y 



SCYPHULA 



Hydrocorallina. 



JTrachylincc 



H YD RULA 



Frc;. 101. Diagram illustrating the mutual relationships of the Coelentemtu. 



By many authors the Sponges have been looked upon as so 

 closely related to the Ccelenterata to be capable of being 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, 

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

 derm, the fixed mode of life, and, associated with it, the tendency 

 to form compound structures or colonies by a process of budding. 

 In addition, the occurrence in both groups of the planula and 



