160 D*ARCY POWER. 



the same position in Vertebrata. I have traced the silver-out- 

 lines on the surface of the following organs — 1, The long 

 muscles of the body-wall (elongate cell-outlines with undula- 

 tions) ; 2, ovary (polygonal ceil- outlines) ; 3^ testicular sacs (large 

 polygonal cell-outlines); 4, outer neurilemma (polygonal); 5, 

 intersegmental septa (interlocking undulated outlines.) 



In Plate X. fig. 9 is represented a portion of the tissue 

 which holds the coils of the nephridia to the intersegmental 

 septa. This tissue is not merely an epithelium, but forms an 

 actual membrane comparable to the more delicate examples of 

 mammalian omentum. It is typical ''retiform" tissue. Con- 

 siderable tracts of this fenestrated membrane may be isolated 

 and spread on the glass-slide for observation — that is to say, 

 pieces measuring the one-sixth of an inch in diameter. The 

 '' fenestrse " vary greatly in size, as in the omentum, measuring 

 from the -rr'-rotli to the — outli of an inch in diameter. In the 

 specimens from which the drawing was made the nuclei of the 

 cells were well preserved, but the silver-outlines were only here 

 and there present in such strength as is shewn in the plate. 



Endothelium of Blood-vessels. — The specimens of membran- 

 ous septa from the earthworm, treated with silver, frequently 

 shewed blood-vessels in which cell-outlines were marked out 

 with all the distinctness which one is accustomed to see in 

 silver-preparations of the vertebrate mesentery. 



In Plate X. tigs. 7 and 8, I have represented some of these 

 vessels, and in figure 5, a pair of the characteristic vascular 

 globules or swellings so abundant on the renal organs (ne- 

 phridia) are drawn, one shewing its silver-outlined endothelium. 



The endothelial outlines of the two parallel-running vessels 

 drawn in fig. 7 are remarkable on account of the differences 

 which they present, the cell outlines being in the one case such 

 as to indicate elongated cells few in number, whilst in the other 

 case the cells are shorter, more numerous, and more variously 

 interlocked. Such differences are observed in vertebrata be- 

 tween small arteries and veins as between these two vessels, the 

 more elongate epithelium characterizing an artery. It is im- 

 portant to obtain in this way distinct evidence of the differentia- 

 tion of the vascular trunks of the Earthworm into arterial and 

 venous, afferent and efferent — a diflerentiation which has some- 

 times been doubted, but which it would be necessary to assume 

 in the case of such an elaborate excretory organ as the ne})hri- 

 dium from which these two vessels are taken, even had we not 

 the structural evidence of the fact which I now submit. 



I have not been able in the smaller vessels, such as those 

 drawn in fig. 7, measuring -pJ^-^th of an inch or less in 

 diameter, to determine the existence of any muscular coat or of 



