266 EDWARD A. MINCHIN. 
Ectoderm.—I am more in the dark about this layer than 
about any other part of the sponge. All that was made out 
with certainty was as follows:—In surface views, after fixing 
with osmic and removal of the spicules with acid, one sees at 
intervals patches of black granules (figs. 7 and 8). Some of 
these are isolated, but most are continuous with the wall of a 
pore, which also appears granular, either all round or only on 
one side (see fig. 8; in fig. 7 there are no pores). In macera- 
tion the cells appear as seen in fig. 20, where the clear space 
doubtless represents the nucleus. Fig. 21 represents a single 
pore macerated out. Each pore appears to be formed of a 
single ectoderm-cell. In sections the superficial ectoderm-cells 
sometimes appear as little heaps of granules (fig. 22), but are 
in general very hard to make out. On the other hand, the 
granular walls of the pores are easily seen. It is possible that 
in the fully formed sponge wall the ectoderm-cells may to a 
certain extent degenerate into a cuticle-like structure.| I may 
1 Topsent, in his most important memoir on the Clionide already cited, 
finds that in these forms, the most contractile sponges known, and also in the 
genera Reniera and Halichondria, it is the flattened epithelial cells of 
the ectoderm and endoderm clothing the canals, sphincters, &c., that are the 
real contractile elements (pp. 24—27, 96, &c., and p. 122). He terms 
them “cellules de revétement,’’ and states the following important fact: 
* Sur ces points [i.e. on the papille of Cliona] les cellules de revétement 
n’auraient pas de raison d’exercer leur contractilité; aussi y sont elles rem- 
placées constamment par une cuticule incolore d’apparence anhiste”’ (p. 26). 
The author then cites the observations of Kolliker and Schulze as to a similar 
cuticle in Cacospongia and Euspongia, &c. Now in our Ascetta the only 
parts where contraction could take place is round the pores and round the 
openings of the sieve membrane, and it is precisely in these places that I 
find the cellular nature of the “‘cellules de revétement”’ most distinct. In 
other places it has, I believe, degenerated, as Topsent finds. This author’s 
work was not known to me when I made my observations. Metschnikoff, in 
his ‘‘ Anatomisches tiber Ascetta”’ (‘Spongiologische Studien ui, Zeitschr. f. 
wiss. Zool.,’ xxxii, pp. 8358—3862, Taf. xxii), finds the ectoderm very distinct 
in A. blanca, primordialis, and clathrus; and is astonished that earlier 
writers (Haeckel, Oscar, Schmidt, and Keller) could not see it. He figures 
distinct epithelial cells (figs. 9—11), but these are from Olynthus forms, 
in which he finds the ectoderm “‘ noch starker und auffallender ausgebildet als 
bei den obenerwahuten Tarrusformen.” It seems to me probable that 
