20 



THE TURBELLARIA 



been carefully worked out by Vejdovsky for Bothrioplana (Fig. 

 VII. 5) ; the flame cells are in two series, one dorsal and another 

 ventral. In the Acoela it is doubtful whether an excretory system 



FIG. VII. The Chief Plans of Arrangement of the Main Canals of the Excretory System in 

 Rhabdocoels (Figs. 1-4), after v. Graff; Alloiocoel (Fig. 5), after Vejdovsky; and Triclad 

 (Fig. 6), after Ijima. 



1. Derostoma presents the most primitive arrangement ; each of the paired excretory pores 

 (c) leads into a main canal or duct (a), whence capillaries (b) are given off, terminating in 

 flame cells. By the approximation of the ducts posteriorly, and subsequent fusion of their 

 hinder ends, the condition represented in 2, for Plagiostoma, arises, where the median posterior 

 pore leads into a short common duct. A further step results in the total fusion of the two 

 ducts along their whole extent, as in 3, Stenostoma, and a specialisation of the terminal part 

 of the common duct gives rise to a contractile bladder (d). 



4.Mesostoma. Here the two excretory pores have become involved in the invagination 

 which forms the pharyngeal sac (g), in which lies the pharynx (/). 



5. In Bothrioplana there are two median excretory pores, one anterior (a), and one near the 

 centre of the body (g), both on the ventral surface. The latter appears to have been derived 

 from the condition seen in Mesostoma, by the approximation and fusion of the transverse 

 excretory ducts. The main canals on either side have the normal condition, being recurved 

 at the anterior end, and the four canals so formed appear to have effected a secondary com- 

 munication with the exterior, a, anterior pore leading into a duct (fo) with contractile wall ; 

 dorsal canal (d) entering the dorsal or recurrent limb (e) of the main lateral canal, ventral branch 

 (v) enters the main canal (c), which is connected with a plexus of capillaries along its whole 

 extent, the right and left canals being connected at the posterior end of the body by the 

 plexus in. The lateral plexus gives off at intervals short branches (xx) which pass towards 

 the surface of the body ; but no pore has been detected. From this lateral network there 

 arise mesially a definite number of branches, each terminating in a flame cell (h), of which 

 five lie in front of, and four behind the pore (gr). A second series of flame cells (i), seven in 

 number, are carried by branches from the dorsal stem ; the latter terminates in a plexus (j) 

 above the pharynx, n, the brain ; p, p' , the two pairs of ciliated pits. (The two median 

 plexuses are represented too strongly.) 



6. Plan of a Triclad. a, main canal opening at d, d to the exterior ; there are four or 

 five pores not strictly symmetrical in arrangement. Each main canal gives rise to a branch 

 (e) which gives origin to a plexus (/) in the substance of the pharynx (according x to Chichkoff). 

 g is the eye. 



