REPORT ON THE TETRACTINELLIDA. 
305 
complete. The margins of the plates are incurved towards the poral faces, rendering them 
more concave than they would otherwise be. Where the oscular faces of two plates have 
come in contact in course of growth, they have grown together. The height of this 
fragment is 75 mm., the thickness of the plate from 8 to 10 mm. Bowerbank’s specimen, 
with a height of 188 mm., presents a thickness of from 10'5 to 12'5 mm. 
Most of the specimens illustrate in a very striking manner the local death to which 
sponges, and especially Lithistid sponges, are liable ; the greater part of some of the 
fragmentary plates consists of dead skeleton, the remainder of complete and apparently 
healthy sponge. In one case the wall having everywhere decayed around a circum- 
scribed patch which still remained alive, the latter, prevented from extending laterally, 
has increased in thickness, so that it is 1 mm. thicker than any of the normally grown 
specimens with which it is associated, and, of course, much thicker than the rest of the 
plate to which it belongs. 
The whole exterior surface of the sponge is invested with an epithelium, on the under 
surface of which spirasters are thickly scattered. In a tangential section one sees below 
this the cladomes of the dichotrisenes, with the cladi of various sizes extended in every 
direction, and not regularly mapping out poral areas, as is the case with many other 
sponges provided with similar spicules. Between the cladi, often in the angle of the 
deuterocladi, tubercular processes are seen extending directly upwards from the under- 
lying desmas. On the oscular surface the dichotriaeiies extend up the sides of the. 
oscular cones, but do not proceed far beyond the margin of the summit, which is 
usually from 0'4 to 0'5 mm. in diameter, and bears in the centre the oscule, which is 
from OT to 0‘3 mm. in diameter. The epithelium, with its associated spirasters, extends 
across the annular space which separates the oscule from the desmas, and is continued 
downwards as a lining to the excurrent tube. On the poriferous surface the desmas are 
absent from numerous special areas, each of which is perforated in the centre by a single 
pore, from 0’02 to 0‘06 mm. wide; sometimes no poral aperture can be distinguished, the 
position of the pore, however, remains clearly indicated by what appears to be a dense 
aggregation of spirasters ; this is, however, an optical delusion, due to one’s looking down 
into the poral canal, the sides of which are covered with spirasters underlying the 
epithelium. The dichotrigenes are not regularly arranged around the poral area, and 
their cladi do not extend quite close to its margin. 
The ectosome (PL XXXIV. fig. 13) is about 0'8 mm. thick on both faces of the 
sponge. It consists of a gelatinous matrix full of round or oval empty vesicles, from 
0‘02 to 0’0316 mm. in diameter; occasionally a small quantity of granular protoplasm 
is seen investing a part of the interior of the vesicle, and within this, bulging it out on 
one side, is a spherical or oval vesicle, about 0'004 mm. in diameter, containing a central 
granule, together resembling a nucleus with its nucleolus. In other cases several little 
bodies, reminding one of choanocytes, are seated on the wall. They consist of a basal 
(ZOOL. CHALL. EXP. PART LXIII. 1887.) ElT 39 
