16 



THE CTENOPHORA 



typical Coelentera one or other of the two primary layers, 

 ectoderm or endoderm, retains the functions which in Coelomata 

 are handed over to mesoblast. Hence we find epithelio-muscular 

 cells derived chiefly from ectoderm in Hydrozoa, chiefly from 

 endoderm in Anthozoa. The researches of Metschnikoff, confirmed 

 by Samassa, have shown that a mesoblast is formed in the 

 Ctenophora, that there is no epithelio-muscular system, but that 

 the musculature is wholly derived from the mesoblast. At the 

 same time it must be duly borne in mind that "mesoblast" is 

 nothing more than an embryological segregation of those cells 

 derived in Coelentera or Diploblastic animals from one or both of 

 the primary germ layers which are in Coelomata destined to give 

 rise to the coelom and the tissues of its walls. Greater weight 

 must be attached to the presence of the gastrovascular system in 

 Ctenophora than to the embryonic exhibition of " a mesoblast." 



c.ss. 



FIO. vr. 



Ctenoplnna Kmcalevd-ii , Korotneff (after Willey). it, tentacles; tsh, tentacle sheaths; 

 ctr, sub-transverse costae ; ess, sub-sagittal costae ; st, " stomach " (? stouiodaeum), J, 9, S, A, 

 the four principal lobes of the infnndibulum ; pf, sensory tentacles representing the polar 

 fields ; pg, pigment spots. 



The affinities of the Ctenophora with the Polyclada remain 

 to be considered. These affinities, first suggested by Selenka on 

 embryological grounds, were rendered more probable by the 

 discovery of Coeloplana Metschnikoflii, a form supposed to be 

 intermediate between Planarians and Ctenophora, and were urged 

 with considerable force by Lang (17). The discovery of Cteno- 

 plana Kowalevskii, an animal allied to Coeloplana, by Korotneff (14) 

 served to confirm this view. 



Ctenoplana has recently been rediscovered by Willey (22), 

 who has given a more exact account of its habits and anatomy 

 than Korotneff was able to do. It is a Ctenophore, flattened so 

 much that the principal axis joining mouth and sense organ is 

 extremely short. Hence one can distinguish a dorsal or aboral 



