252 



ZOOLOGY 



SECT. 



In 



and a pair of canals backwards from it, all three giving off branches 

 which again branch. In some Polyclada there are minute pores, 

 by means of which certain of the canals are placed in communica- 

 tion with the exterior. A number of unicellular glands, which 

 probably produce a digestive secretion, open in many Trematodes 



and Rhabdocceles, at the junc- 

 tion of pharynx and stomach- 

 intestine. 



A bilateral nervous system 

 is developed in all the Platy- 

 helminthes. Its elements are 

 nerve-fibres, or rather nerve-tubes, 

 and nerve-cells. The former are 

 tubes containing a very delicate, 

 finely fibrillated, material. The 

 nerve-cells, which are usually 

 bipolar, more rarely uni- or multi- 

 polar, lie in the course of these 

 tubes, with the contained delicate 

 material of which the substance 

 of the cells is in continuity. The 

 degree of development of a cen- 

 tral part of the nervous system, 

 or brain, varies in the different 

 groups ; it is rudimentary in the 

 Turbellaria Accela and in the 

 Cestodes, and best developed in 

 some Polycladida and some Mono- 

 genetic Trematodes. It consists 

 of numerous nerve-tubes which 

 here converge from the various 

 parts of the body and pass across 

 from one side to the other, to- 

 gether with a central mass of 

 matter similar to that contained 

 in the nerve-tubes, and a number 

 of nerve-cells. It is situated in 

 the anterior portion of the body, 

 almost invariably in front of the 

 mouth. When the peripheral 

 part of the nervous system is 



best developed, as it is in the Polycladida, the Tricladida, and 

 some Trematodes, there are three pairs of longitudinal ncrvc- 

 cords running backwards from the brain throughout the body, 

 connected together by frequent transverse connecting nerves, or 

 commissures. To these there are sometimes super-added fine net- 

 works or plexuses of nerves, situated superficially under the dorsal 



FIG. 202. General plan of the structure of 

 a Triclad. en. brain ; e. eye ; </. 

 germariuni ; i. median limb of the in- 

 testine ; ?' 2 . right limb ; i s . left limb ; 

 In. longitudinal nerve-cord ; m. mouth ; 

 od'. oviduct ; ph. pharynx ; t. testes ; 

 tc. tentacles ; vd.vas deferens ; u. uterus ; 

 <J ejaculatory duct ; 9 vagina ; $ 9 

 common genital aperture. (After Von 

 Graff.) 



