CTBNOPHORA. 



53 



Regarding the stomodseal folds the discrepancy, that they are paired organs in Ctenophores, 

 a continuous ringwall in Polyclads, cannot be removed at present. But the strong development of 

 the folds in Tjalfiella^ Ctenoplana and Coeloplana is very suggestive of the "krausenformiger Pharynx" 

 found in the more primitive Polyclads; especially in Coeloplana, where the walls of the pharynx are 

 "thrown into a great number of folds" (Abbott, p. 51) the condition would seem to be nearly as in the 

 primitive Polyclads; unfortunately Abbott does not give any information as to whether they are 

 really paired structures as in Tjalfiella or go all round the pharyngeal wall. 



Through the observations on Mertensia ovum recorded below my attention was called to the 

 fact that the stomodseum of Ctenophores really consists of two divisions, viz. a larger, lower part, 

 comprising the stomodaeal folds — the pharynx s. str. — and a small upper part, consisting of a narrow 

 canal, forming the connection between the pharynx and the infundibulum; this latter part, which I 

 designate as the oesophagus, is always strongly compressed in the sagittal plane. In Mertensia it is 

 unusually long and conspicuous, while in other typical Ctenophores it is only quite short, which 

 accounts for its not having been distinguished hitherto. In Tjalfiella, Coeloplana and Ctenoplana it 

 is very conspicuous; (it is designated as the "stomodseal canal" by Abbott). (Comp. the note p. 22). 

 The same two parts are also to be distinguished in the Polyclads. Lang does not treat this oeso- 

 phagus specially, designating it only as the "inner mouth"; he sees therein only the opening in the 

 "diaphragm" through which the pharyngeal cavity is in connection with the stomach. That it is 

 really a canal comparable to the oesophagus of Ctenophores is, however, evident enough; I need only 

 refer to such a figure as Taf. 28, fig i of Lang's Monograph. Unfortunately he does not give any 

 information as to whether it is compressed in the one or other plane; it might be expected to be 

 compressed in the sagittal (longitudinal) plane. Wishing very much to have this rather important 

 question settled, I asked for some specimens of a pair of the more primitive forms of Polyclads from 

 the Zoological Station at Naples, viz. Thysanozoon Brochii Gr. and Stylochus neopolitanus Lang. On 

 removing the dorsal skin over the pharynx in Thysanozoon it was very easily seen that the inner 

 opening of the oesophagus is really a longitudinal slit, as it ought to be according to the 

 theory; in Stylochus the opening could not be exposed clearly by direct preparation, but horizontal 

 sections showed that it is likewise here a longitudinal slit. — The diaphragm, the wall separating 

 the stomach from the pharyngeal cavity in Polyclads, and through which the oesophagus passes, is, 

 of course, also represented in Ctenophores, though generally quite small on account of the compression 

 of the pharynx. In Mertensia it is distinct enough (Comp. fig. 11 d.), on account of the widening of 

 the pharynx, and in Tjalfiella it is even more developed than in any Polyclad; in fact the whole wall 

 separating the pharyngeal ("suboral") cavity from the infundibulum and transverse canal must be 

 regarded as the homologue of the diaphragm of Polyclads (comp. PI. VI, figs. 2 and 9) ; in Ctenoplana 

 it must evidently be very nearly the same condition, and also in Coeloplana the diaphragm must 

 doubtless be very distinct, judging from the description of the pharynx. 



The new facts brought to light thus all tend to strengthen the homology of the gastrovascular 

 system in Polyclads and Ctenophores; the only differences of any weight to be noted being the 

 histological character of the epithelium in the pharyngeal sac (ciliated in Ctenophores, non-ciliated 

 in Polyclads) and the stomodseal folds, paired organs in Ctenophores, unpaired in Polyclads — 



