Ill 



PLATODES—GASTRO-GANAL 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 Cmloplana. 

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

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

 among the lihabdomlidcB in a few Acmla and Mesostomidm. 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 



^ 



r^e 



"inEiiimimMiBijjj 





C afe 



jmii im'>^Bun i i ] Mi i » 'i i,ajim]iiLi^mjj i]sniin!srm^ 



<llttH;nVW.[IHH'HI'l "fTT.1Tn'T7mT n-TTt-trmr m 'rrTii„m i^f^ ,.^ 



<^ dm. . /d£n. 



He 



Pig. 102.— j1-D, Diagrammatic representation of tie Pltaryngeal Apparatus of tlie Turljel- 

 laria. A, Of Convolnta ; B, of Mesostoma ; C, of Planooera ; H, of ProstMostomnm. d«, Dorsal 

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

 geal pouch ; lih, pharynx ; d, intestine ; dep, intestinal epithelium ; s, oesophagus ; p, parenchyma ; dt, 

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



RhahdoccelidcB in many Accela, and .also in Mesostomidm, Probosddea, and 

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

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

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

 few Leptoplanidce among the Polydada, all Tridada, and many 

 PihahdoccdidcB (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 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 Oonvolutidce, the Microsto^nidce, and 

 Macrostomidce among the Pihahdocmlidce, is distinguished by its great 

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



