TYPE PLATYUELMINTHES. 



129 



with nerve-cells lying between the inner ends of the ectoderm- 

 cells as well as with sensory cells resembling in general form 

 those already described as occurring in the Cnidaria. It must 

 not be understood, however, that the ganglion-cells are limited 

 in their distribution to the lower layer of the ectoderm and 

 the brain ; on the other hand, they are scattered along the 

 nerve-cords which arise from the brain, the Platyhelmmths 

 presenting in the structure of their nervous system a condi- 

 tion intermediate between the diffuse arrangement of the 

 ganglion-cells seen in the Cnidaria and the more perfect ag- 

 gregation occurring in higher types. 



An excretory system of branching tubes traversing the 

 mesoderrn parenchyma and opening to the exterior is also 

 present. It consists usually of two main tubes, nephridia, 

 from which numerous branches arise, terminating in blind 

 funnel-like extremities (Fig. 70, f) lying in the meshes of the 

 parenchyma. Each funnel (Fig. 

 78, J3] is closed by a single cell 

 (tc), from which there projects 

 into the tube a bundle of cilia 

 (fl), and which, from the resem- 

 blance of the motion of these cilia 

 to a flame flickering in the wind, is 

 known as aflame-cell. The larger 

 tubes are lined by a layer of 

 cells which seem, in certain cases 

 at least, to be ciliated, but the 



smaller branches consist of a_ 



JIG. 70. EXCRETORY SYSTEM OF 



series of cells succeeding one THE ANTERIOR PORTION OF THE 



another in a single row, the canal BODY OF Planaria montana (after 



running through the centres of CHICHKOFF >- 



the cells and being thus iutra- 



cellular. The tubes throughout the entire system contain 



a fluid in which particles resembling guanin in their behavior 



to reagents have been seen, and there is little room for doubt 



but that the tubes have an excretory function. 



Finally, a complicated reproductive apparatus (see Figs. 

 68-70) is present, the Platyhelmmths being for the most part 

 hermaphrodite. The testes consist of from two to many 



