326 Excretory Organs dbc. o/" Tetrastemma graecense. 



cilia, although it was only now and then that I could perceive 

 this distinctly in prepared sections. 



The knobbed terminal organs, the outer surface of which 

 is smooth, either vest with a relatively broad base upon the 

 end-canalicules, which not infrequently ramify, or else the 

 portion turned towards the canalicule is drawn out like a 

 stalk ; in the former case they are more bulky in shape, in 

 the latter more elongate. Each terminal organ is closed at 

 its free end by two flame-cells (more rarely one), from which 

 delicate strands of plasma radiate into the mesenchyraa; in 

 the formation of its wall several — as it seems to me from three 

 to five — cells take part, which in structure resemble those of 

 the end-canalicule. In general it may be said that the end- 

 canals open into the main canals not directly, but by means 

 of the connecting portions ; yet I have also seen a direct 

 communication between the two. 



The number of the excretory pores could not be determined 

 with certainty in the living animal, but on examining the 

 series of transverse sections from two individuals it was found 

 that in the first case there were five pores on each side and 

 in the second six on the one side and three on the other. 

 The pores are always situated dorsally and at irregular 

 distances one from another ; in the case of the «pecimen, too, 

 which possessed five pores on each side, those of the right 

 side did not correspond with those of the left. 



At the points at which excretory pores occur the excretory 

 organ is applied closely to the dermo-rauscular tube and the 

 short efferent duct bores its way straight through dermo- 

 niuscular tube, basement membrane, and epithelium. 



A more intimate relation between the nephridia and the 

 blood-vessels, such as has been demonstrated by Burger in 

 the case of marine jMetanemertines, especially Drepanopliorus^ 

 does not exist in Tetrastemma graecense. 



The blood-vascular system consists of three trunks, two 

 lateral vessels and a dorsal vessel. The latter opens in front 

 into the right lateral vessel close behind the brain, behind into 

 the anal commissure between the two lateral vessels. 



The wall of all these vessels is formed by an internal endo- 

 thelium, a muscular sheath, and an outer layer of mesenchyma 

 cells arranged in epithelial fashion. Now between the endo- 

 thelium and muscular sheath large cells of hemispherical 

 form and peculiar structure thrust their way ; in the condition 

 of diastole these cells stand out like knobs from the wall of 

 the vessel, but in that of systole they project into its lumen. 

 Since two cells of this kind always lie du-ectly opposite to 

 one another, or nearly so, they are able in systole to close the 

 lumen almost completely and prevent a backward '^o\s of the 

 blood. 



