TYPE PLATYHELMINTHES. 



135 



-vi 



The mouth is situated at various regions of the body iu 

 different forms, being anterior in Microstoma, while in Meso- 

 stoma (Fig. 72) it is situated at the middle of the ventral sur- 

 face. The walls of the pharyn- 

 geal pouch {"pK) may be quite 

 simple, as in the Accela which 

 possess a pharynx, or may pre- 

 sent a muscular thickening form- 

 ing a bulbous pharynx, but no 

 further complexity occurs, al- 

 though in certain forms, such as 

 Frorhynchus, the pharynx is capa- 

 ble of being proti'uded from the 

 mouth, acting probably as a 

 delicate tactile organ. 



The nervous system (w) is 

 essentially similar to that of the 

 AUoiocoela ; two or four eyes {oc) 

 frequently occur, though otocysts 

 are wanting, while the ciliated 

 depressions on the side of the 

 head supposed to be olfactory in 

 function occur in Microstoma and 

 Frorhynchus and allied forms. 



The excretory system consists 

 occasionally of a single nephri- 

 dium with numerous branches 

 which open near the posterior 

 end of the body, but more usually 

 two main tubes are present open- 

 ing near the middle of the body 

 either directly to the exterior or 

 into the pharyngeal pouch {Meso- 

 stoma), though in some cases they 

 unite near the posterior end of the 

 body into a single tube which '»^ = yolk-gland, 

 opens to the exterior by a single median pore ( Vortex). 



The reproductive system presents considerable variation 

 in the structure of the female apparatus, but the testes (t) are 



Fig. 72. — A Rhabdocoiloub Tur- 

 BBLLAiiiAN, Mesostoma splendi- 

 dura (after von Graff). 



at ■■ 

 atg ■■ 



d : 

 n ■■ 



oc ■■ 



ov 



P 



p?i 



sr 



i 



atrium. 



atrial gland. 



intestine. 



brain. 



eye. 

 : ovary. 



penis. 

 : pharynx. 



receptaculum seminis. 



testis. 



