v PHYLUM PLATYHELMINTHES 263 



as backwards. In the Rhabdocoeles 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. 

 Eyes 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 Rhabdocoeles 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 developed 

 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 

 stotoliths 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 concerned with the maintenance 

 of equilibrium. Ciliated pits which appear to be sensory are 

 developed in some Rhabdocoeles in the head region. 



The only vascular system present in the Platyhelminthes is 

 the system of water-vessels (protonephridia) which are commonly re- 

 garded as performing an excretory function. The arrangement of 

 these, the mode of ending internally of the finest branches, and the 

 way in which the system communicates with the exterior, vary greatly 

 in the different groups One or more main longitudinal trunks give 

 off branches which subdivide to form a system of* minute inter- 

 lacing branches or capillaries. In little spaces at the ends of the 

 capillaries are a number of highly characteristic structures the 

 ciliary flames. Each ciliary flame consists of a bundle of vibratile 

 cilia ; typically each is situated in the interior of a cell the 

 flame-cell (Fig. 217) terminating one of the capillary branches. 

 But there are some cases in which there are several flames in each 

 flame-cell. The finer branches, and in some cases the larger trunks 

 also, are intracellular, and are to be looked upon as perforations 

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

 larger trunks are intercellular, being lined by an epithelium of 

 small cells. This system of water-vessels opens on the exterior 

 in a variel^Tof different ways : sometimes it opens by a number 

 of minute pores ; sometimes, as in the Liver-Fluke, there is a 



