CTENOPHORA. 



43 



plana the tentacle axis and the stomachal axis are equipolar; bnt if we consider about which axis the 

 paired structures are situated, we are simply forced to acknowledge that the plane of the tentacles 

 corresponds to the sagittal plane - - in other words that the tentacle axis of Ctenoplana and Cteno- 

 phora correspond to the longitudinal axis of Bilateralia". -- Lang's theory "rests in the first instance 

 on the assumption that the pinnate tentacles of Ctenophores and Coeloplana are homologous with the 

 sensory tentacles of Polyclades"; but it must be emphasized that "under no circumstances and from 

 no point of view are the tentacles of Ctenoplana bilaterally disposed, but they are biradially 

 disposed". It is true that when Ctenoplana creeps, its tentacles look as if they were transversely dis- 

 posed "and it may seem difficult to imagine an ancestor of bilateral animals with an unpaired tentacle 

 in front and an unpaired tentacle behind. But the point is that we have got to imagine this, because 

 in the animals with which we are dealing there are no such relations as anterior and posterior, right 

 and left" (p. 335). Willey accordingly finds it much more probable that the nuchal tentacles of Poly- 

 clads are the homologues of the "sensory tentacles" of Ctenoplana, which are again homologous to the 

 polar fields of typical Ctenophores. 



The researches of Willey lead him to the conclusion that Ctenoplana must be regarded as 

 "an ancestral form, and not, as some Zoologists seem to suppose, a highly modified creeping Cteno- 

 phore . . . That the Planarians and Polyclades in particular have close affinities with the Ctenophora 

 there can be no doubt, but it is very much open to question, whether the former are derived from 

 the latter. The view that the Polyclades are so derived seems a reversal of the natural order of 

 events, which point to the littoral fauna as the origin both of the pelagic and the abyssal fauna". 

 Considering the bilateral symmetry of Polyclads, and especially their well developed nervous system, 

 they can scarcely be imagined to be derived directly from amorphous forms (like Trichoplax] "but rather 

 from animals which possibly, like Ctenoplana, possessed a biradial symmetry. Ctenoplana approaches 

 more nearly to a condition of bilateral symmetry than the Ctenophores do, in that it possesses very 

 clearly differentiated dorsal and ventral surfaces. And this is exactly what we should expect to find 

 in the littoral or sublittoral ancestor of such purely pelagic forms as the Ctenophora, the pelagic habit, 

 as is well known, often tending to produce a more or less radial symmetry. On the other hand, a 

 biradial form, like Ctenoplana, possesses the potentiality of assuming a strictly littoral life, in which 

 the ventral surface is the permanent locomotor surface, such an existence leading to a condition of 

 bilateral symmetry, according to well-understood physiological principles". "The ctenophoral plates 

 must have put in their appearance for the first time in some form or other; and although it is at 

 present beyond the limits of our knowledge to explain how they arose, yet it is not right to conclude 

 that the ctenophoral plates of Ctenoplana are degenerate or reduced structures merely because they are 

 smaller than the ctenophoral rows of the Ctenophora. It is a groundless assumption to say that 

 Coeloplana and Ctenoplana are modified creeping Ctenophores. Ctenoplana is an expert crawler, it is 

 expert at hanging on to the surface film of water, and it is indeed an expert swimmer. Everything 

 it attempts it does well in the old primeval fashion, and there is nothing degenerate about it" (p. 338). 



Coeloplana and Ctenoplana thus are neither Ctenophores nor Planarians, but represent a sepa- 

 rate order, the Archiplanoidea, equivalent to the orders Turbellaria, Trematoda etc. From the 

 Archiplanoidea are derived both Ctenophores and Plathelminthes. In the same way the Anthozoa and 



