THE MARINE TURBELLARIA. 



307 



The Prosthiostomidae resemble the Eiiryleptidae, from which family they are probably 

 derived, in possessing a tubular pharynx. They have no marginal tentacles, but the absence 

 of these is probably secondary, perhaps a consequence of their elongated form. The structure 

 of their male apparatus is very extraordinary, quite unlike that of other Polyclads. 



Lastly, the family Diposthidae described by Woodworth [20] from the barrier reef of 

 Australia is probably closely allied to the Eiiryleptidae, but unfortunately no account is given 

 of the pharynx. Avihlyceras is regarded by von Plehn [12] as intermediate between the 

 Pseudoceridae and Euryleptidae. 



The accompanying tree illustrates the presumed relationships of the Cotylea diagrammatically. 



Pseudoceridae 



Pericelis- 



Anonymus- 



Amblyceras 



Dipcsthidae 



-Euryleptidae 



Prosthiostomidae 



Cotylea 



The characters of the terminal parts of the female apparatus give strong support to 

 the view that the Cotylea constitute a perfectly natural group. In none of them is the 

 vagina prolonged backwards beyond the point at which the uteri open into it to form an 

 'accessory vesicle' or receptaculum seminis ; its walls are never developed to form a muscular 

 'bursa copulatrix,' and the shell-glands always open into it close to the gonopore. Whatever 

 specialization of the female apparatus has been developed has occurred either in connection 

 with the oviducts themselves as in Oligocladus or with the uteri as in Pericelis and Diposthus. 



[The presence of an accessory vesicle in Enantia, as well as the arrangement of the 

 eye-spots and the absence of a sucker form a combination of characters which forbid us to 

 refer it, as Benham does [3], to Cotylea. In the possession of a gut anastomosis it does 

 not by any means stand alone amongst the Acotylea, witness Leptoplana pardalis, Planocera 

 villosa, Eustylochus elUpticus.] 



Cotylea and Acotylea. The Acotylea can be distinguished broadly from the Cotylea 

 by the fact that the former as a rule possess an accessory vesicle, a continuation of the vagina 

 behind the point where it receives the uterine opening, though this is by no means invariably 

 the case, e.g. the genus Hoplopkma. But the most striking and most constant distinguishing 

 feature (apart from the sucker) of the two divisions is afforded by the tentacles, when they are 

 present. Those of the Cotylea have already been dealt with, and are of course universally 

 marginal. 



In the Acotylea marginal tentacles never occur; when they are present they lie dorsally 

 immediately over the brain. In the Leptoplanidae and I believe in Enantia these tentacles, 

 which in the Planoceridae always carry eye-spots, have been lost, the eye-spots connected with 

 them have nevertheless been retained, at any rate in most cases. In the Cestoplanidae so far 

 as we know there are no traces of tentacles distinguishable (cf the other elongated family. 



