268 ZOOLOGY sect. 



helminthes. Its elements are nerve-fibres and nerve-cells. The 

 nerve-cells, which are usuall}- 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 development of a 

 central part of the nervous system, or brain, varies in the different 

 groups ; it is best developed in some Polycladida and some Mono- 

 genetic 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 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 nerve-cords running backwards 

 from the brain throughout the body, connected together by 

 frequent transverse connecting nerves, or commissures. To these 

 there are sometimes superadded 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 as backwards. In the Rhabdo- 

 coeles and some of the Trematodes the whole system is simpler, 

 and the number of longitudinal cords fewer. In the Cestodes 

 there are two principal longitudinal trunks which run throughout 

 the length of the body, and are connected together in the head by 

 commissures, variously thickened to form ganglia representing the 

 brain of other Platyhelminthes. 



In addition to the tactile cones of some Trematodes and the 

 sensory cilia of the Turbellaria, already referred to, the sensory 

 organs of the Platyhelminthes are the eyes and the statocysts. 

 £yes occur in the Turbellaria and some Monogenetic Trematodes, 

 but are wanting in the Digenetic Trematodes and in the Cestodes. 

 In some of the Polycladida they are extremely numerous, collected 

 into groups over the brain, and frequently arranged also round the 

 margin of the body. In the Ehabdocoeles and Monogenetic 

 Trematodes they are much less numerous — usually two to four. 

 In some cases each eye simply consists of a pigment spot ; to 

 this may be added a refractive body. When most highly de- 

 veloped the eye is still of very simple structure, consisting of 

 a cup formed of one or more pigment-cells enclosing refractive 

 bodies {rods), and having nerve-cells in close relation to it with 

 processes (nerve-fibres) passing to the brain. The statocysts are 

 sacs containing statoliths of carbonate of lime. The function of 

 these bodies, which occur only in a small number of the Turbellaria, 

 is unknown ; there is no sufficient evidence that they are organs of 

 hearing ; it is more likely that they are organs connected with 

 the maintenance of the equilibrium. Ciliated pits which appear to 

 be sensory are developed in some Rhabdocoeles in the head region. 



