Ill 



PLATODES GASTEO-CANAL SYSTEM 



139 



The position of the mouth is in no animal race so extraordinarily 

 variable as in the Platodes. We hold that the original position of the 

 mouth is the central one, in accordance with the views set forth in the 

 general remarks, and with its position in Ctenoplana and Codoplana. 

 This position is found only in Ttvrbdlaria, i.e. among the Polydada in 

 the families of the Anonymidce, Planoceridce, and most Leptoplanidce, and 

 among the Rliabdoccelidce in a few Accela and Mesostomidce. In the 

 remaining Platodes the mouth is placed either more to the front or 

 to the back, without in any case reaching the extreme anterior or 

 posterior end of the body. The mouth lies somewhat further forward 

 than the middle among the Polydada in the Pseudoceridce, among the 



FIG. 102. A-D, Diagrammatic representation of the Pharyngeal Apparatus of the Turbel- 

 laria. A, Of Convoluta ; B, of Mesostoma ; C, of Planocera ; D, of Prosthiostomum. de, Dorsal 

 body epithelium ; ve, ventral body epithelium ; o, mouth ; dm, entrance to intestine ; pt, pharyn- 

 geal pouch ; ph, pharynx ; d, intestine ; dep, intestinal epithelium ; s, resophagus ; p, parenchyma ; dt, 

 dorsal oesophageal pouch ; vt, ventral cesophageal pouch ; ms, muscular lamella. 



Rhabdoccelidce in many Accela, and also in Mesostomidce, Proboscidea, and 

 Plagiostomida. It is found near the anterior end of the body in the 

 Euryleptidcv and Prosthiostomidce, among the Polydada, in many Rhabdo- 

 ccela, and all Trematoda. It lies somewhat behind the middle in a 

 few Leptoplanidce among the Polydada, all Tridada, and many 

 Rhaldoccelidce (especially in the Monotidce). It is met with near the 

 posterior end of the body in the Cestoplanidce among the Polydada, and 

 in many Plagiostomidce among the Alloioccda. 



The structure of the pharyngeal apparatus is very varied in the 

 Platodes. We can only briefly describe the chief types. 



The pharyngeal apparatus of the Convolutidce, the Microstomidce, and 

 Macrostomidce among the Rhaldoccelidce, is distinguished by its great 

 simplicity and its embryonic condition (in most Accela it is even 



