TURBELLARIA. 



213 



(Yungia, Cydoporus) the enteric diverticula open to the exterior. 

 The internal wall of the enteron is some- 

 times ciliated. 



In the Acoela the pharynx leads into 

 the central parenchyma, in the vacuoles 

 of which digestion takes place. 



The excretory organs are the draining 

 system of the parenchyma. They also 

 sometimes extend into the endoderm, 

 which is really the central part of the 

 parenchyma. A general account of what 

 is known of these organs has already been 

 given. To that may now be added that 

 there is generally a main trunk opening 

 externally by one or several openings 

 (Fig. 172) on the dorsal, or on the ventral 

 surface, or even into the pharynx-sheath; 

 that this main canal gives off secondary 

 canals which may branch and even anas- 

 tomose ; that the secondary canals receive 

 the finest canaliculi, which come direct 

 from the terminal cells the flame-cells 

 as they are called. Flame-like cilia and 

 smaller cilia may project into these canals 

 at different parts of their course. 



The whole system is said to be intra- 

 cellular, and without well-marked walls. 

 It can only be made out in the living 



FIG. 168. Alimentary canal and 

 nervous system of Mesostomum 

 Ehrenbergii (after Graff). G the 

 two cerebral ganglia with two 

 eye-spots; St the two lateral 

 nerve - trunks ; D alimentary 

 canal with mouth and pharynx. 



Phg 



Dst 



Sch 



Qm 



Dvm 



FIG. 169. Transverse section of Planaria polychroa, passing through the pharynx (after 

 Leuckart and Nitsche). D intestine ; Ph pharynx ; Phg cavity of pharynx ; Pht sheath of 

 pharynx ; Ln lateral nerve-trunks with commissures Co connecting them, and lateral 

 branches Sn ; T testes ; Od oviduct ; Dst yolk-glands ; Dvm dorsoventrally running muscular 

 fibres ; Qm transverse muscular fibres ; Sch mucous glands opening externally at the edge of 

 the ventral surface ; Vd vas deferens. 



