288 WILLIAM BLAXLAND BENHAM. 
conspicuous sac, subtransparent when fresh, and contrasting 
with the pinkish mass of tubules. Muscles traverse it in all 
directions ; and round the external pore (ne. 0.) they are concen- 
trated into a set of circular muscles, acting as a sphincter, 
outside which are radial muscles. Its cavity is lined by 
columnar cells, between which and the muscles is some con- 
nective tissue with a poor supply of blood-vessels. 
In one of the ordinary nephridia (for the shape of the vesicle 
varies somewhat in various regions, figs. 21, 25, 26) the exter- 
nal pore is not situated at one end of the vesicle, but very 
much nearer the rosette of tubules; whilst the vesicle is pro- 
longed outwards, more correctly dorsally, and ends in an obtuse, 
blind end (c. v.), near the mid-dorsal line above the alimentary 
tract. At the opposite end the vesicle is rapidly constricted, 
and it is here that the rosette of tubules is attached ; these are 
well supplied with blood-vessels which give the rosette a pink 
appearance when fresh. 
(2) The rosette of tubules (ne. ¢.) consists of from ten to 
fifteen loops. Each “loop” is a tubule bent upon itself in 
a U-shape, the apex of the y being free, and the two limbs of 
the loop spirally wound round each other (fig. 21). The con- 
stitution of one of these tubules is seen in figs. 31, 32, 33, 
Pl. XVI dts. Passing along the inner side of a loop is an 
intracellular lumen with rather a greater diameter than the 
others, this I call the ‘‘ main lumen” (/). It pierces a series 
of granular cells, the whole of whose diameter it does not 
occupy, so that the wall is fairly thick and noticeably granular, 
the granules appearing to have a somewhat radial arrangement 
(fig. 33). Outside this lumen there runs, parallel with it, a 
“secondary lumen” (/'). This lumen occupies nearly the whole 
diameter of the cells through which it passes, so that its walls 
are thin and are not granular. These two are closely bound by 
connective tissue, amongst which run the “ smaller lumina” 
(/’), forming a copious network round the other two lumina 
(fig. 31); like that of the secondary lumen their walls are thin 
and non-granular, their diameter varies, but is smaller than 
either of the other two lumina. The main lumina, near the 
