11 
appears next and now the contents stand out freely into the ven- 
tral furrow, the cellular matrix and muscular investment having 
disappeared on the exposed side (fig. 33). The muscular invest- 
ment is then lost and in the following sections a portion of the 
ventral groove, between the ciliated cells of the foot region and 
the spieulae of the integument is formed by the cuticular contents 
of the coeca just mentioned. Here these contents stand out more 
or less sieve-shaped and in their histological appearance look very 
much the same as the interspicular substance of the integument 
into which it unnoticeably merges. This layer becomes thinner 
gradually (fig. 34) and only disappears at a certain distance be- 
hind the region where the rectum and the genital organs severally 
open into the posterior anal cavity. 
When studying the secretion oft these coeca with stronger powers 
it becomes evident that the transparent substance inside them is trav- 
ersed by straight and prismatic cavities, running parallel to each 
other, rather strongly inclined backwards and most of them opening 
to the exterior in the sieve-like end plate ch (fig. 33). Moreover there 
is a central space cb (fig. 32) left open in the midst of the honey-comb- 
like secretion. This inner cavity is apparently lined with a layer in 
which nuclei may be here and there detected. In it another very 
large number of the straight cavities open. T'hese cavities alluded to 
are not all empty but again filled with a thread-like secretion bt 
of extreme delicacy, which is secreted by the epithelium. These 
fine thread-like bundles find their way to the exterior at the point 
where the cavity along which the bundle travels opens, be this 
in the central canal (fig. 32) or in the sieve-like plate (fig. 33 
and 34). 
The threadlike secretion induced me to compare this organ in a 
preliminary notice published elsewhere (Zoologischer Anzeiger n? 70) 
with a byssusgland of very primitive construction and as yet I see 
no interpretation which would be more in accordance with the 
facts observed. However I do not as yet wish to express by 
this name any fixed opinion as to its actual homology with the 
true byssus-gland of other Mollusks; more especially because the 
