20 A. CO. OUDEMANS. 
cease. This lacunar region I have called the esophageal 
region. The blood-spaces have now changed their form 
into one which in transverse section is circular or ellip- 
soidal, so that now the name of vessels can be applied. 
Internally they are clothed with the cellular epithelium, next 
follows a hyaline basal layer and externally a layer of circular 
muscular fibres. I have nowhere observed a longitudinal layer 
of fibres. The region which follows the cesophageal one I will 
call the nephridial region, because we meet here with organs 
visibly belonging to that type of excretory apparatus of the 
Evertebrata, which Ray Lankester has united under the name 
of nephridia. 
Above the blood-vessel, directly applied against it in a slanting 
direction, and only separated from it by a thin wall, hes a 
second large longitudinal canal, the nephridial canal (see figs. 4, 
25, 26, 27). Its walls are similar to those of the blood-vessels. 
Thus the thin wall that separates them consists of five layers: 
epithelium, hyaline basal tissue, circular muscular fibres, 
hyaline basal tissue, epithelium. Indeed, a highly primitive 
nephridium, which is, as it were, a part of the blood-vessel 
separated from it! Now,in the whole nephridial region a 
spongy organ lies in the blood-vessel, placed on its outer wall, of 
which to my regret I could not make out sufficient histological 
details, at least none which I would venture to communicate as 
yet. This organ, which presents itself as a spongy gland, I will 
call the nephridial gland (see figs. 4, 25 to the left and to the 
right, and fig. 26 to the right). The nephridial canal has at its 
front end an open communication with the blood-vessel (see 
fig. 4 and fig. 25 on the right). Also at the other end where its 
lumen gradually becomes smaller (see fig. 4 and fig. 26 to the 
left). The nephridial canal further at intervals has open com- 
munications with the nephridial gland (see fig. 4 and fig. 25 on 
the left, and fig. 26 on the right). The lumen or the lumina of 
this gland are always separated from the lumen of the blood- 
vessel by a very thin wall. I have clearly noticed two excretory 
ducts, one on each side, by which the lumen of the nephridial 
canal communicates directly with the exterior, consequently also 
