ON THE ANASPIDACEA, LIVING AND FOSSIL. 535 
thick and conspicuous; and it is thrown into marked 
sinuosities of outline; this membrane stops abruptly when 
the mid-gut passes over into stomo- or proctodeeum. 
Exteriorly to the basement membrane is a thin submucous 
layer with flattened darkly-staining nuclei (PI. 12, fig. 5). 
The second dorsal diverticulum of the mid-gut is by 
far the largest. Histologically (Pl. 12, fig. 6) it is simply 
formed as a pocket from the dorsal mid-gut epithelium, and 
its cells are elongated and columnar with very numerous 
nests of flattened special gland-cells. There can be no doubt 
about the function of this diverticulum; it is simply a diges- 
tive gland which pours its secretions into the alimentary 
canal. 
The remaining portion of the mid-gut lying between the 
second and third diverticula is of a simple structure, and is 
evidently chiefly of an absorptive nature (Pl. 12, fig. 7). 
The epithelium is composed of short columnar cells with 
striated outer borders; the basement membrane, charac- 
teristic of the mid-gut, is still to be observed, while the 
submucosa forms a thick reticular layer in which large 
homogeneously staining nuclei are embedded. ‘There are no 
eland-cells in this region. 
The third dorsal diverticulum is exceedingly small, 
and is composed of columnar cells and numerous nuclei 
embedded in a common protoplasm. There are no special 
gland-cells. A fair number of mitoses can always be observed 
in this diverticulum, though not so many as in the first 
diverticulum ; its function may be similar to that suggested 
for the first, viz. to keep up a.supply of epithelial cells for 
the lining of the alimentary canal. The intestine behind the 
third diverticulum is proctodeum, being lined internally with 
chitin, which is thrown into numerous folds. The epithelium 
is columnar and hyaline, being very similar to that of the 
stomodeeum. 
It remains to describe the liver. This organ is composed 
of very numerous slender tubes, as many as thirty being 
often present, which open ventrally into the pyloric division 
VOL. 03, PART 3.—NEW SERIES. 37 
