236 MR. A. T. WATSON ON THE STRUCTURE 
between the end of the torus and bundle of sete of that 
side of the segment to which the gland belongs. There are 
sometimes seven pairs of these glands, one pair to each of 
the first seven segments, but those of the third segment 
may be either rudimentary or non-existent. These are the 
thread-glands (‘ glandes filiéres’’) of Claparéde (6), the struc- 
ture of which has been fully worked out by Gilson (20). Their 
function is to secrete—by exudation from the epithelial cells of 
the wall of the gland (20. p. 317, & 29. p. 402)—a very thick 
viscous liquid containing bundles of exceedingly fine colourless 
threads, which are used by the animal in the formation of the 
membranous lining of its tube, but not, as some writers have 
suggested, for the attachment of its external stony coveriny. 
The secretion for this latter purpose is supplied by the “Lippen- 
organ ”’ (/.0., figs. 1, 3, 7 & 9). The esophagus, which is almost 
as long as the “ thoracic” segment, descends directly from the 
mouth, and is followed by the intestine, which, with a series of 
swellings and constrictions, runs straight to the anus. 
De Saint-Joseph notes (29. p. 403) that in the third and 
fourth “abdominal” segments the intestine is green owing 
to the glands which cover it. The alimentary canal passes 
through the segmental septa, and in addition thereto is sup- 
ported throughout by a dorsal and ventral mesentery. For the 
greater part of its length it is surrounded by the dorsal vessel, 
which forms a peri-intestinal sinus. This sinus bifurcates and 
leaves the esophagus on entering the thoracic region. Hach 
branch then goes forward, and, after skirting the nearest group 
of branchial processes, descends, and, reuniting with its fellow, 
gives rise to the ventral vessel. Numerous short ceca arise 
from each side of this vessel, forming round ampulle, of which 
de Saint-Joseph states (29. p. 403) that he has counted as many 
as forty in the third abdominal seement. The blood is red. 
The septa, which are membranous and traversed by fine muscular 
fibres, have been specially studied by Gilson (27). They may 
completely separate each segment from its neighbour, but by 
means of valves in the interseemental membranes interecommuni- 
cation may be established. 
These valves (Pl. 23. fiy. 5) are of two kinds, namely, a simple 
slit, making a “flap” valve (f-v.), opening like a door towards 
the anterior, which is always situate dorsally; and a sphincter 
valve (s.v.), which is placed ventrally. I have been fortunate 
