126 
MR. H. N. MOSELEY ON THE 
living layer the elements of the network take a direction more or less vertical to the 
surface. The horizontally directed main canals and their branches lie near the under 
surface of the layer on a level with bases of the zooids. The histological structure of the 
hydrophyton is shown in Plate 3. fig. 16. The canals and vessels forming the network 
are composed of an ectodermal layer, with a membranous layer developed beneath it, 
and of endodermal lining. 
The ectodermal layer consists, in the greater part of the network, of fusiform cells 
with a finely granular appearance, and a well-defined oval nucleus, but with the cell- 
boundary often very indistinct. These cells form in some places much thicker layers 
than in others. Towards the most internal part of the hydrophyton they become entirely 
lost, their place being taken on the surface of the thin-walled cyst-like innermost 
elements of the network by a thin layer of structureless protoplasm (Plate 3. fig. 16, B). 
Narrow strings of this ectodermic protoplasm (Plate 3. fig. 16, S) cross over here and 
there between adjacent vessels of the innermost part of the network, being possibly the 
remnants of effete vessels. The ectoderm covering the parts of the network near the 
surface is much thickened and modified, a large proportion of its cellular elements 
being there found converted into the parent-cells of thread-cells, and being thus inflated 
and occupied by thread-cells in all stages of development. At the actual surface the 
ectodermal cells undergo still greater modification, forming a superficial layer of 
elongate prismatic transparent cells, which shows on the surface a series of irregularly 
hexagonal areas corresponding with the summits of the cells. These cells contain oval 
nuclei and thread-cells of both kinds in various stages of development. The most 
superficial film showing the hexagonal areas separates often in osmic-acid prepa- 
rations as an exceedingly thin membrane, as is shown on the right-hand side of Plate 3. 
fig. 16. There is some uncertainty as to the exact structure of the superficial layer of the 
ectoderm. The figures represent what, after a careful investigation, was concluded 
to be the arrangement existing. The layer is seen well only in preparations from speci- 
mens hardened in osmic acid. The lateral boundaries of the prismatic cells were 
never seen well defined, but the polygonal areas corresponding with their summits were 
seen well in various preparations. It could not be demonstrated with certainty that 
this layer extends uniformly all over the external surface of the hydrophyton. It is 
extremely transparent, and difficult to trace in preparations viewed from the surface, 
over the cavities caused by the removal of the hard parts by decalcification. The exact 
arrangement of the superficial layer in its connexion with the mouths of the calicles 
and zooids is also somewhat uncertain. The layer certainly is prolonged into the 
calicular cavities, and contains the mass of large oval thread-cells which surrounds each 
zooid. In most preparations the zooids are far retracted through the action of reagents, 
and the mouths of the calicles are closed above by a layer of tissue, which shows a 
radiate striation or slight plaiting around a very small circular central orifice, which 
orifice leads down a short tubular cavity formed by the superficial layer drawn every- 
where inwards to the retracted zooids of the calicle. The layer of tissue thus contracted 
