TTPE PLATYHELMINTHES. 



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 Platyhelminths 

 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 

 mesoderm 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 funrid (Fig. 

 78, B) is closed by a single cell 

 (tc), from which there projects 

 into the tube a bundle of cilia 

 (Jl), and which, from the resem- 

 blance of the motion of these cilia 

 to a flame flickering in the wind, is 

 known as a, flame-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 

 series of cells succeeding c 

 another in a single row, the canal 

 running through the centres of 

 the cells and being thus intra- 

 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 Platyhelminths being for the most part 

 hermaphrodite. The testes consist of from two to manj 



Fig. 70. — Excektobt System op 



one .jjjj; Anterior Portion of the 



Body of Planaria montana (after 



CeiCHKOFF). 



/ = funnel. oc = eye. 



