CLASSIFICATION OF POLYCLADIDA 



17 



Fig. 8 shows that, starting with a member (A, D) of each 

 division, in which the month is almost in the middle of the 

 ventral surface, and the brain and sense organs somewhat 

 remote from the anterior end, we find in the Acotylea a series 

 leading to an elongated form (Cestoplanidae), in which the 

 mouth, pharynx, and genital pores are far back near the hinder 



g m : ph m m 

 ph 



Fig. 7. Diagrammatic vertical longitudinal sections : A, Of Prosthiostomum (type of 

 Cotylea) ; B, of Leptoplana ; C, of Cestoplana (types of Acotylea). (After Lang. ) 

 These figures illustrate the changes which follow the shifting of the mouth from 

 a central position (B) to either end of the body, br, Brain ; dphm, "diaphragm " ; 

 gm, true mouth ; Ig, openings of lateral gut-branches ; m, mouth ; mg, main-gut 

 or stomach ; mgbr, median gut -branch ; ph, pharynx ; ph.m, aperture in pharyn- 

 geal fold ; phs, peripharyngeal sheath ; sc, sucker ; <$, male, and ?, female, genital 

 aperture. 



end of the body ; while in the Cotylea the series leads simi- 

 larly to the elongated Prosthiostomatidae, in which, however, 

 the pharynx and external apertures are in the front part of 

 the body. This view of the morphology of the Polyclads is 

 due to Lang, and is based on the assumption that the more 

 radially -constructed forms (Pig. 8, A, D) are the primitive 

 ones. 



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