in 



PL A TODES—XER VO US S YSTEM 



U7 



spread out in the anterior part of the body. Commissures between 

 the longitudinal nerves appear in larger numbers only in the Monotidce 

 among the Alloiocmla, which show near relationship to the Tridada in 

 other points also. In Mesostoma Ehrenbergii there is a tranverse com- 

 missure behind the pharynx ; in Microstoma lineare two other nerves 

 besides the longitudinal nerves proceed backwards from the brain, 

 surround the pharynx, and unite behind it. 



In the Acicla, according to earlier investigators, a nervous system 

 was wanting ; but lately a somewhat complicated nervous system has 

 been proved to exist. It consists of 2 ganglia lying one behind 

 the other in the anterior part of the body (the front ganglion 

 being the smaller), and six longitudinal nerves, 2 inner, 2 middle, 

 and 2 lateral which run along the edge of the body. The 2 

 inner arise from the posterior ganglion, the 2 middle and the 2 outer 

 from the front ganglion. All these 6 nerves are connected by trans- 

 verse commissures which go off at right angles and themselves again 

 anastomose. 



The nervous system of the Trematoda (Fig. 106, p. 143) is closely 

 connected with that of the Tridada and Pohjdada. It consists of a 

 brain, from which proceed, besides small nerves which run to the sides 

 and the front, 6 nerve trunks running backwards, viz. 2 dorsal, 2 

 inner ventral, and 2 outer ventral or lateral. The 2 ventral are 

 connected together and with the lateral by more or less numerous 

 transverse commissures, as are the lateral with the dorsal, and the dorsal 

 again together ; these tranverse 

 commissures may again anastomose 

 with each other. 



The nervous system assumes 

 this form especially in ectoparasitic 

 Trematoda (Tristoma) and in Distoina 

 isosfomum. In many other Trema- 

 toda, however, perhaps in most 

 species of Distonia, the commissural 

 system appears to have degenerated ; 

 and of the 6 longitudinal nerves 

 only the inner ventral nerves seem 

 to be strongly developed, as in 

 the Pkhabdocoela. 



The brain lies above the mouth 

 in all Trematoda ; when the mouth 

 is placed in the pit of the oral 



sucker, the brain extends as a transverse bridge over the front part 

 of the pharynx. 



The nervous system of Jmphilina quite agrees mth that of the 

 Distomidce. 



The nervous system of the typical Cestoda (Fig. 107) consists of 

 two lateral longitudinal trunks, which pass through the whole body 



Fig. 107.— Nervous System in the Scolex 

 of a Tapeworm (Taenia serrata) after Niemic. 

 Of the 8 weaker longitudinal nerves only the 

 4 on one side (dui, dn^) are depicted, sn, 

 Lateral nerves ; gc, chief or cerebral commis- 

 sure. 



