Ill 



PL ATODES NERVOUS SYSTEM 



147 



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

 the longitudinal nerves appear in larger numbers only in the HI<mof/it<i' 

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

 other points also. In Mesostoma Ehrenb&rgii 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 Accela, 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 Avith that of the Tridada and Polydada. It consists of a 

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

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

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

 connected together and Avith 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 

 Avith each other. 



The nervous system assumes 

 this form especially in ectoparasitic 

 Trematoda (Tristoma) and in Distoma 

 isostomum. In many other Trema- 

 toda, hoAvever, perhaps in most 

 species of Distoma, the comniissural 

 system appears to have degenerated ; 

 and of the 6 longitudinal nerves 

 only the inner ventral nerves seem Ftn . loy.-Nervous system in the scoiex 

 to be strongly developed, as in of a Tapeworm (TsBnlaBerrata) after Niemlc. 



the Ehahkccda. ": )f the S W6 f " }*f** nerves only the 



4 on one side (KI, rnio) are depicted, sit, 

 The brain lies above the mouth Lateral nerves ; gc, chief or cerebral commis- 



in all Trematoda ; Avhen the mouth sure - 



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 Amplnlina, quite agrees Avith that of the 

 Distomida'. 



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

 tAvo lateral longitudinal trunks, Avhich pass through the Avhole body 



