CTENOPLANA. 721 



in phylogeny. When it is creeping along the bottom of a dish or, back downwards, 

 along the surface film of water, it looks for all the world like a Polyclade Turbellarian. 

 The fitful extrusion of the opaque white tentacles which have serpentine intrinsic 

 movements of their own, reminds one involuntarily of the behaviour of the Nemertine 

 with its proboscis. On the other hand, when Ctenoplana undertakes to swim, it does 

 not do so by the graceful skirt-like evolutions of the Polyclade, but it assumes a helmet 

 or Pilidium shape, depressing the two halves of the body until they assume a vertical 

 position like the flaps of a bonnet, and then a set of organs, called ctenophoral plates, 

 which are utterly foreign to Turbellarians but distinctive of a group of hyaline pelagic 

 organisms, the Ctenophora, come into play. 



The ctenophoral plates which, as I have said, are the swimming organs of Ctenoplana, 

 occur as eight small oval areas on the dorsal surface. Each plate is beset with a 

 limited number of transverse rows of powerful cilia agglutinated together at their bases 

 so as to form a membrane upon which the cilia are inserted like the teeth into the 

 back of a comb. 



These characters seem to indicate that Ctenoplana occupies a central position midway 

 between the pelagic Ctenophora and the littoral Plathelminthes. This, however, is the 

 simple view of the matter and there are many practical obstacles to its adoption. 

 Some zoologists contend that Ctenoplana is to be interpreted as a specially adapted 

 creeping Ctenophore, but as a result of my observations, which unfortunately left many 

 questions still unanswered, I advocated its ordinal autonomy, chiefly on the ground of 

 its possession of 'distinct male genital ducts opening to the exterior on the dorsal 

 surface of the body 1 . 



The principal features of interest in the organisation of Ctenoplana, features which 

 cannot fail to mark it out for special discussion whenever the interrelations of the 

 lower orders happen to be adjudicated, may be summarised as follows : — 



I. Abstract positive features. 



a. Dorsiventral differentiation. 



b. Biradial symmetry. 



c. Main axis passing between the median ventral mouth and the median dorsal 

 sensory apparatus. 



II. Abstract negative features (involved in the preceding but worthy of special 

 tabulation). 



a. Absence of antero-posterior differentiation. 



b. Absence of ambidextral differentiation (there is no right and left). 



III. Concrete positive features. 



a. Ctenophoral plates. 



b. Semi-opaque, pigmented integument. 



c. Paired pinnate tentacles (Greiftentakeln). 



d. Paired aboral, ciliated, sensory papillae. 



1 Willey, A., "On Ctenoplana." Quart. J. Micr. Sc, Vol. xxxix., 1897, pp. 323—342. PI. 21. 



