CTENOPHORA 157 



the assumption of such relations cannot be altogether re- 

 jected. The similar position of the central nervous system 

 at the anterior pole of the body in the Ctenophora and 

 many worm larvas, the production of the mesoderm as a 

 separate germ-layer in the form of four mesodermal bands 

 arranged crosswise, and the high state of development of 

 the mesenchymatous tissue, appear to argue for such an 

 assumption. First of all, there are, as we shall see, many 

 features agreeing with the development of the Turbellaria. 

 Accordingly there do seem to exist certain relationships 

 between the Ctenophora and the hypothetical ancestral form 

 of the Bilateria. Nevertheless we hesitate for many 

 reasons to imagine the latter to be precisely a Cfcenophoi^e. 

 In contrast to the Turbellaria, which by the retention of a 

 uniform coat of cilia recall primitive conditions, the 

 Ctenophora represent a side branch of the phylogenetic tree, 

 which has developed independently along one line, but 

 which scarcely furnished the basis for the direct develop- 

 ment of higher animal forms. 



In the remarkable forms Cccloplana Metschnikouii and Ctenoplana 

 Kowalevskii, forms directly intermediate between the Ctenoi^hora and 

 Turbellaria were thought to have been recognized (Nos. 13 and 15). 

 However, to us they appear to present only peculiarities that can readily 

 be exj^lained from the typical structure of a Ctenojahore as the result of 

 adaptation to a creeping mode of life. The similarity to the Turbellaria 

 would then rest upon mere analogy. Such an explanation is admissible, 

 for even among the true Ctenophora some forms have the jjower of 

 adhering to firm surfaces and of creeping about by means of the broad- 

 ened foot-like mai-gins of the mouth (Lampetia), so that the starting- 

 point is here given for development in this direction. The fact that 

 with the degeneration of the ribs (ciliate bands) the general ciliation 

 secondarily came again into prominence ought not to be very surprising, 

 for Chun and E. Hertwig have shown that remnants of a general ciliation 

 are retained in the adult condition of the Ctenophora also. 



It should be mentioned that in the origin of the four entodermal sacs, 

 in the presence of the four mesodermal bands, in the development of the 

 ribs on four adradially placed ectodermal thickenings, etc., there is mani- 

 fested a distinct tendency to express its quadriradial structure. Probably 

 the biradial structure of the Ctenophora has been developed from the 

 quadriradial by the different development of each pair of opi^osite radii, 

 so that the biradial structure does not represent the simplest condition 

 of the radial type, but corresponds to a derived condition. 



