supporting lamella

HISTOLOGY AND DEVELOPMENT OF MYRJOTHELA PHRYG1A. 519
In the Triclads and Polyclads, on the other hand, the same
problem has been solved in a rather different fashion. The
development of spaces round the gut is not the most obvious
fact, but the gut itself ramifies into a network of tubes which
penetrate to every part of the body.
The Structure of the Endoderm. — Leaving these
general considerations, we will pass at once to a consideration
of the structure of the endoderm in Myriothela.
The mouth is bounded by a thin membranous lip, or hypostome,
which is exceedingly muscular and sensitive, and is
probably of considerable use in seizing the prey. It is figured
by Allmann and Hincks in a condition of extension. In
fig. 20 it is shown as it appears when the animal is retracted.
In the lip the supporting lamella is either reduced to the
thinnest film or is entirely absent. It is always absent from
about half the breadth of the lip from the free edge. The
ectoderm of the external surface is rich in nerve-cells, and
overlies a well-developed layer of radial muscles. At the free
edge there is no loss of histological continuity, the ectoderm
being continued on to its under surface for a certain distance,
and having the same structure. Passing downwards towards
the attachment of the lip, there appears at the base of this
ectodermal epithelium a more and more defined layer of
highly vacuolate cells, which are the first commencement of the
endoderm. In other words, the ectoderm for a short distance
overgrows the endoderm, thus forming a distinct zone of mixed
character.
At the point where the lip merges into the tentacle-bearing
region the ectoderm, as a distinct structure, has finally disappeared,
and we have the arrangement shown in fig. 14.
Three kinds of cells may be distinguished: (1) a superficial
layer of elongated cells, staining well and uniformly
with picro-carmine. Each possesses a nucleus and nucleolus
in its basal portion, which is tapering and wedged in between
the subjacent cells. In favorable preparations these cells
appear covered with short, fine cilia. These are, however, not
always visible, mainly because the animal in dying usually