306 F. F. LAIDLAW. 



the dorsal papillae of Thysanozoon hrocchii, it may be questioned whether their function is 

 similar. In T. brocchii on the one hand these diverticula may assist in the aeration of the 

 tissues. In Planocera armata on the other hand it is difficult to suppose that they can have 

 this function ; I think it possible that they may rather be concerned in some way with nitro- 

 genous secretion. 



The function of the dorsal papillae of certain species of Thysanozoon, which contain no 

 gut-diverticula, is also obscure. In Thysanozoon plehni the curious plug-like mass of tissue at 

 the upper end of the diverticulum is to some extent suggestive of being connected with the 

 function of excretion. 



Relationships of the Cotylea. In dealing with Pericelis, Thysanozoon and Prosthio- 

 stomum in the systematic part of this paper I have given a fairly full account of the male 

 genital apparatus of each, so that it is possible to compare and contrast the three chief forms 

 that this organ assumes in the Cotylea. The arrangement of the eye-spots in Gestoplana mal- 

 divensis and Pericelis byerleyana completely round the margin of the body is so far as at 

 present known only paralleled in Anonymus virilis, Gryptocelis, and perhaps in Heteroplana 

 newtoni. I believe that this marginal distribution of the eye-spots is to be regarded as a 

 primitive character derived from a radially symmetrical ancestor. 



In the Cotylea other indications of a radial structure are most strongly developed in 

 Anonymus virilis. Such indications are the sub-central mouth and the numerous male organs 

 grouped around the single female aperture. The presence of organs in this species bearing 

 a general resemblance to nematocysts is probably also an ancestral character, so that if we 

 adopt the usual theory and assume that the Polycladida are derived from an ancestor possessed 

 of radial symmetry we are justified in regarding Anonymus as nearer to this ancestral form 

 in certain respects than are the other Cotylea. 



Intermediate between Anonyvius and the higher Cotylea stands Pericelis byerleyana, the 

 most interesting type in Mr Gardiner's collection. This approaches Anonymus in having mar- 

 ginal eyes and a sub-central mouth. Like this genus too it has a median indentation of the 

 anterior margin, whilst the structure of its penis approaches that of the latter more closely 

 than that of other Cotylea. On the other hand the possession of tentacles, a single penis 

 and uterine vesicles, ally it rather to the higher Cotylea, such as the Pseudoceridae and Eury- 

 leptidae, whilst the extreme folding of the margin, the wide separation of the small tentacles 

 and the backward position of the genital apertures serve to distinguish it sharply from all 

 its allies. 



The marginal tentacles of Pericelis are of particular interest. They are of a simple and 

 primitive character and indicate clearly the manner in which these organs have arisen in the 

 Cotylea by the concentration of the marginal eyes on a pair of specialized folds of the margin. 

 We can hardly doubt that this concentration has accompanied the acquisition of polarity in 

 the organism in its development from a radially symmetrical ancestor. The fact that Anonymus 

 has no tentacles, suggests the probability that this remarkable form is far more primitive 

 than other Cotylea in that it has never acquired such organs. 



A further development of the tentacles accompanied with the loss of the marginal eyes, 

 save of such as have come into connection with the tentacles, gives us the condition found in 

 the Pseudoceridae and Euryleptidae. The former family has a more primitive type of pharynx 

 than the latter, in which it is tubular ; in the Pseudoceridae it is not unlike that of Anony- 

 mus or Pericelis. 



