PHYLUM PLATYHELMiNTHES 



253 



integument, or on both dorsal and ventral surfaces. Sometimes 

 nerves run forwards from the brain as well as backwards. In the 

 Rhabdocceles and some of the Trematodes the whole system is 

 simpler, and the number of longitudinal cords fewer. In the 

 Cestodes there are two longitudinal trunks which run throughout 

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

 a commissure which represents 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 octocysts. 

 Et/es occur in the Turbellaria and some Moiiogenetic Trematodes, 

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

 In some of the Polyclada they are extremely numerous, collected 

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

 margin of the body. In the Rhabdocceles and Moiiogenetic 

 Trematodes, they are much less numerous usually two or 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 of pigment enclosing refractive bodies, and having nerve- 

 cells in close relation to it. The otocysts are sacs containing 

 otoliths 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, as their name is meant to 

 imply, they are organs of hearing. 



The only vascular system present in the Platyhelminthes is the 

 system of water-vessels which are commonly regarded as perform- 

 ing an excretory function. The arrangement of these, the mode 

 of ending internally of the finest branches, 

 and the way in which the system com- 

 municates with the exterior vary some- 

 what in the different groups. A series 

 of main longitudinal trunks give off 

 branches which subdivide to form a system 

 of minute interlacing branches or capil- 

 laries. In little spaces at the ends of the 

 capillaries are a number of highly charac- 

 teristic structures the ciliary flames. Each 

 ciliary flame consists of a bundle of vibra- 

 tile cilia ; typically each is situated in the 

 interior of a cell the flame-cell (Fig. 

 203) terminating one of the capillary 

 branches. The finer branches, and in 

 some cases the larger trunks also, are 



intra-celhdar, and are to be looked upon as perforations in 

 linear rows of elongated cells. In the Cestoda, at least the 

 larger trunks are inter-cellular, being lined by an epithelium of 



FIG. 203. A flame-cell. /. 

 processes ; k. termination 

 J i.if capillary ; n. nucleus 

 f. vacuoles ; /cr\ ciliary 

 flarne. After Lang.) 



