226 
THE YOYAGE OF H.M.S. CHALLEHGEE. 
II. Microscleres. 6. Sterraster XXL fig. 21), spherical, depressed, 0 064 to 0'07 
mm. in diameter. 
7. Somal sjpheraster (PI. XXL figs. 22, 23), a large spherical centrum produced into 
numerous short obtusely conical spines; usually 0 'OIOS mm. in diameter, sometimes as 
much as 0‘031 mm. ; the spines of forms larger than the average are often expanded 
at the end, and much resemble the spines of a sterraster. 
8. Somal spheraster (PI. XXL fig. 24), minute, of very variable characters, usually 
a well-marked centrum, with thick conical or rod-like actines, abruptly truncated at the 
ends or strongylate ; the actines may increase in length and become sharply pointed, 
or they may be reduced till the aster passes into a mere globule. Diameter O'Ol mm. 
9. Choanosomal oxyaster (PL XXI. figs. 25, 26), a small centrum, and smooth, 
slender, sharp, conical actines ; usually numerous, sometimes reduced to four or five ; 
usually about 0'012 mm. long, sometimes as much as 0’017 mm. ; total diameter from 
0'0276 to 0'039 mm. 
Colour. — Y ello wish -white. 
Habitat. — Port Jackson, June 1874; 6 to 15 fathoms. 
Remarks. — There are two specimens of this sponge, the largest 22 mm. in diameter. 
The cortex (PI. XXL fig. 29) is about 0*318 mm. thick. Externally it is invested by an 
epithelial layer, beneath which lie the minute spherasters (No. 8) ; a single layer of spher- 
asters (No. 7 ) embedded in collenchyma succeeds ; then follow the sterrasters ; the inner- 
most layer of the cortex is very thin, and consists chiefly of fibrous tissue, the cladomes of 
the dichotrisenes lie within it, and in the areas between the cladi the endochones are situated. 
The chones are of the usual character, the pores of their roofs are about 0*016 to 0*05 mm. 
in diameter, or on an average 0*025 mm. ; the sphincter dividing the ectochone from the 
endochone is well developed. The chones lead into incurrent canals which descend radially 
into the choanosome, several uniting some distance below the cortex into a large canal 
running concentrically (PL XXL figs. 27, 28). The relations of the smaller branches 
of the incurrent and excurrent canals to one another and the flagellated chambers is 
shown in thin sections with diagrammatic clearness (PI. XX. fig. 22), and the incurrent 
system can be traced from beginning to end ; the excurrent canals have, however, only 
been traced in one direction, i.e., towards the flagellated chambers ; whenever an 
excurrent canal is traced towards the cortex it is found to end by branching into flagel- 
lated chambers, and in no case has it been traced into connection with a chone ; only 
one-half of the small specimen was cut up into serial slices, and it is possible, therefore, 
that the excurrent chones will be found in the remaining half. 
The flagellated chambers measure about 0*0276 mm. in length by 0*0316 mm. in 
breadth ; they lie immediately adjoining the sides of the ultimate branches of the 
incurrent canals, with which they communicate by prosodi, about 0*012 mm. in diameter, 
