262 



ZOOLOGY 



SECT. 



Zn 



give off numerous branches. In the Polycladida (Fig. 215) there 

 is a central cavity from which numerous branching canals are 

 given off. In the Tricladida (Fig. 216) one median canal passes 

 forwards from the pharynx, and a pair of canals backwards from 

 it, all three giving off branches which again branch. In some 

 Polycladida there are minute pores, by means of which certain 



of the canals are placed in commu 

 nication with the exterior. A num 

 ber of unicellular glands, which pro- 

 bably produce a digestive secretion, 

 open in many Trematodes and 

 Khabdocoeles at the junction of 

 pharynx and intestine. 



A bilateral nervous system is 

 developed in all the Platyhelmin- 

 thes. Its elements are nerve-fibres 

 and nerve-cells. The nerve-cells, 

 which are usually bipolar, more 

 rarely uni- or multi-polar, lie in 

 the course of these fibres, with 

 which the substance of the cells is 

 in continuity. The degree of de- 

 velopment of a central part of the 

 nervous system, or brain, varies in 

 the different groups ; it is best 

 developed in some Polycladida and 

 some Monogenetic Trematodes. It 

 consists of numerous nerve-fibres 

 which here converge from the 

 various parts of the body and 

 pass across from one side to the 

 other, together with a central mass 

 of fine fibrils, and a number of 

 nerve-cells. It is situated in the 

 FIG. 2i6.~ 7 Generai plan of the structure anterior portion of the body, almost 

 germa^iumft' median & i\mb of YhV in- m variably in front of the mouth, 

 testme; i 2 .' right limb; ? 3 . left limb; Where the peripheral part of the 



In. longitudinal nerve-cord ; m. mouth ; * . , 



od. oviduct ; ph. pharynx ; t. testes ; nervOUS System IS DCSt developed, 

 a; v. vitellaria; vd. vas ag it ig j n ^ p o ly C l a( Jida, the 



Tricladida, and some Trematodes, 

 there are three pairs of longitudinal 

 nerve-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 integument, or on both dorsal and ventral 

 surfaces. Sometimes nerves run forwards from the brain as well 



