Ill 



PL A TODES GA STRO-CA NA L S YSTEM 



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 Ccelophni'i. 

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

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

 among the Ehabdoccelidce in a few Acoela 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 



cfe 





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, cesophagus ; p, parenchyma ; dt, 

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



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

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

 EwryleptidcB and Prosthiostomidce, among the Polydada, in many lUuibdo- 

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

 few Leptoplanidce among the Polydada, all Tridada, and many 

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

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

 in many Plagiostomida'. among the Alloioccela. 



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 Microstomida>, and 

 MacrostomidcB among the BhdbdocoslidcB, is distinguished by its great 

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



