ON THE DEVELOPMENT OF LICHENOPORA VERRUCARIA. 91 
a new ovicell superposed on the old one (cf. p. 75). Judging 
by the analogy of the walls of the alveoli, I think it possible 
that the floor of these blister-like spaces may ultimately be 
absorbed, and the ovicell thereby enlarged. 
It is hardly possible to examine figs. 8 and 9 without feeling 
convinced that there must be a layer of living tissue outside 
the roof of the ovicell. Although I have not obtained complete 
evidence with regard to the morphological character of this 
layer, its presence can be demonstrated in vertical sections of 
stained colonies which have not been decalcified. In pre- 
parations of this nature a film of stained nucleated tissue can 
be seen, passing over the outer side of the calcareous parts of 
the upper surface of the colony, and thus covering the upper 
side of the roof of the ovicell, and extending up the outside of 
the zocecial tubes to the orifices of the latter, where the tissue 
passes round to join the part of the body-wall which is invagi- 
nated into the orifice of each zowcium. The ridges on the 
outer side of the zocecia are as a matter of fact undeveloped 
septa, which can continue their growth under certain circum- 
stances. It is thus natural to find that the outer wall as well 
as the inner wall of the young and active zocecia should be 
covered with living tissue. 
Where the protoplasmic layer passes over the convex surface 
of an alveolus, or up the uniform surface of a zocecium, it is 
usually tightly stretched over the adjacent calcareous matter. 
But in places where it crosses a groove between two alveoli, 
the layer usually appears as a tangent of the two curved surfaces. 
Hence one sees in sections a more or less triangular space, 
roofed only by a delicate membrane, at the junction of two 
alveoli. It is obvious that this is in accord with the arrange- 
ments shown in fig. 9. 
The calcareous part of the upper surface of a Lichenopora 
is thus not the outermost layer; and it becomes a question 
how far it corresponds with the “ectocyst”? of some other 
Polyzoa. In the Entoprocta, the Phylactolemata, and the 
Ctenostomata, in which calcification does not usually occur, 
the outermost layer is a mere cuticle; and it is to this layer 
