EPITHELIOID MEMBRANES IX MONAXONID SPONGES 569 
but terminate abruptly." The figure (pL 34, fig. 22) given by 
SoUas indicates plainly that the canal lining in Pachymatisma 
is similar to that in Stylotella. Dendy ('93) finds that the cells 
of the canal epithelium in certain calcareous sponges, Grantessa, 
Sycon, Vosmaeropsis, are sometimes separated from one another 
by wide intervals. He regards this appearance as due to contrac- 
tion, the cells having pulled away from one another. They may 
still remain connected, he says, in places by strands of proto- 
plasm. Some of Dendy's figures (pi. 14, figs. 60, 62, 64) suggest 
that in these sponges too the canal lining may be of the type 
found in Stylotella. 
Note — I am fortunately able to refer to a publication by Profes- 
sor G. H. Parker^ that has appeared while the foregoing paper has 
been passing through the press. Parker, in the course of an 
interesting physiological study of Stylotella heliophila, one 
of the forms on which my observations were made, touches in- 
cidentally on the histology. Some of his conclusions differ from 
mine. 
He thinks that the dermal epithelium is composed of poly- 
gonal cells. This conclusion rests on a study of sections which 
were apparently vertical to the surface and made from osmic 
material. The dermal layer is so thin that I do not believe it 
possible to learn much of its structure from such preparations. 
The same criticism applies to the conclusion that the dermal 
pores are surrounded by elongated spindle-shaped cells, myocytes, 
which act as sphincters. Surface preparations, such as those 
from which my figures were made, show that the pores are not 
surrounded by cells of this kind. 
Parker finds that the canals are lined with a flat epithelium. 
In addition an abundance of myocytes surround the canals 
(and oscula) arranged like sphincters. ' In some places in my 
preparations they seem to lie directly on the exposed surfaces 
of the canals and cavities that they bound as though they 
were merely elongated epithehal cells" (loc. cit., p. 7). It is 
*G. H. Parker. The Reactions of Sponges, with a Consideration of the Origin 
of the Nervous System. Jour. Exp. Zool., vol. 8, 1910. 
