REPORT ON THE TETRACTINELLIDA. 
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arranged tricliodal protrisencs, the cladomes of which project beyond the aperture, as though 
for defence. The surface of the sponge is irregularly reticulate with numerous low 
anastomosing ridges, surrounding oval depressions, in which the pores are situated. 
These are round or oval openings, about 0'044 mm. in diameter; they lead directly 
into wide incurrent canals, which are crossed by numerous vela (PL I. figs. 12, 14). 
The excurrent canal divides into branches, which are comparatively of large diameter 
up to their ultimate ramifications, and are not provided with vela. 
Fragments of pumice and other foreign bodies are abundant in the incurrent canals, 
and food residues occupy the ultimate branches of the excurrent canals. 
Ectosome (PL I. fig. 15).— The ectosome varies considerably in thickness, from about 
0’06 to 0T18 mm.; it consists of somewhat modified collenchyma, many of the 
coUencytes being elongated into fusiform fibre-cells, which run chiefly more or less 
parallel with the surface ; others in some parts acquire a vesicular character, and 
together with the fusiform fibres produce a fibrous vesicular tissue. Some of the 
eollencytes appear to have lost nearly all the protoplasm of the cell-body, and to have 
become reduced to mere nuclei with their nucleoli ; others are of the ordinary stellate 
branching form, and some of these where they lie near the ectoderm of the outer surface 
send a slender process to it, and give off on the opposite side two other similar 
processes, which descend into the interior ; whether these become connected with other 
cells or not is an open question. It will be seen from this description that the ectosome 
of Tetilla leptoderma is more highly developed than that of Tetilla sandalina; in 
succeeding species of the genus we shall find this development carried further, fore- 
shadowing the corticate type. 
Choanosome (PL I. fig. 13). — The mesoderm is remarkably poorly developed ; it 
consists of collenchyma, in which, besides eollencytes, numerous deeply stained more or 
less oval cells occur, which appear to be contracted amoeboid cells. In some cases the 
flagellated chambers appear to be flattened against one another or against the epithelium 
of the canals, with scarcely a discernible trace of mesoderm between them. 
The flagellated chambers are more or less spherical or ellipsoidal pouches, sometimes 
approaching the form of cylindrical sacs — on an average they measure 0’06 by 0'05 mm. 
but are sometimes larger, in one case an oval section gave 0’081 and 0’067 mm. for 
the two diameters. They communicate abruptly by a wide mouth, 0'032 mm. in 
diameter, with the excurrent canal; and by a single large prosopyle with the incurrent canal. 
The spicular fibres of the body consist of the fusiform oxeas (1), protriaenes (3), 
and somal anatrisenes (5), and near the surface chiefly of the latter, which form 
a diverging sheaf extending beneath the ectosome and through it to the exterior 
(PL I. fig. 15). The smaller oxeas (2) are not aggregated into fibre, but are loosely 
scattered throughout the sponge, with a kind of “ criss-cross ” arrangement, between the 
radiating fibres. At the surface their points project slightly, raising the ectoderm into 
