154 



Platyhelminths. 



its sheath. A long muscle^ the retractor, passes from its inverted 

 end to the sheath, and this pulls in the everted organ {see Fig. 115 D). 

 Its function is defensive and prehensile; it is not connected with 

 the digestive tract. This begins in front and ventrally (behind 

 the proboscis pore) in a slit-like mouth, and runs through the body 

 as a canal, furnished usually with small lateral evaginations, and 

 opening in the anus at the posterior end. 



m 



an 



Dp 



DQ 



Fig. 115. A Sketch of a Nemertine, showing the nervous system, B do. with 

 blood vascular system, C ditto with gut and proboscis, D ditto front end with everted 

 proboscis. Diagrammatic, a Anus, c ciliated groove, aa gut, g poison gland, I lateral vessel, 

 m retractor muscle, r probscis, ro proboscis opening, rs proboscis sheath, s lateral nerve 

 cord, t transverse vessel. — Orig. 



The nervous system is very like the usual flatworm type; the 

 proboscis sheath passes between the cerebral commissures. From the 

 cerebral ganglion of either side arises a lateral nerve, running 

 backwards, and containing numerous nerve-cells; the two cords 

 are often connected by delicate transverse commissures, passing 

 above and below the alimentary canal, and from them many 

 nerves arise. The excretory organs are similar in structure 

 to those of other Platworms. Besides the ciliated grooves and eyes 



