294 
THE VOYAGE OF H.M.S. CHALLENGEK. 
II. Microscleres. 4. Microxea (PL XXXIL fig. 10), fusiform, sharply pointed, surface 
minutely roughened, 0‘05 by 0‘004 mm. 
5. Microstrongyle (PI. XXXI. fig. 11), ellipsoidal or cylindrical, with rounded ends 
and minutely roughened surface, 0‘012 to O'OIG by 0*004 mm. 
Colour. — Yellowish-white. 
Habitat. — Port Jackson, June 3, IST'J; depth, 30 to 35 fathoms. 
Remarhs. — This specimen presents a broad, thick pedicel, terminating in two shallow 
cups ; the growth has not been vertical, but oblique, so that when the sponge is placed 
with its flat base on a horizontal surface, the margins of the cups are nearly vertical. 
The total width of the sponge across the cups is 97 mm., the major and minor axes of 
the margin of one cup measure 56 and 41 mm., the base measures 51 by 42 mm. The 
sponge is completely overgrown, save for one small patch near the base, by an incrusting 
Desmacidine sponge, which appears to have commenced its growth at a time when the 
Lithistid was alive throughout, since beneath the parasite the discotrisenes of the ectosome 
are still preserved ; while had the superficial portion of the host been dead before the 
growth of the parasite, these would have probably dropped off. The discotrisenes are 
also in a quite fresh state, showing no signs of solution, and this also suggests that they 
have not long been dead. 
With the growth of the Desmacidine over the pores of the Lithistid, the latter became 
starved and stifled, and in consequence began slowly to die away, till at the time it was 
dredged all that remained alive was a small central patch, 15 by 6 mm. in area when cut 
open, and a small portion near the base which had escaped the general covering up. With 
the decease of the Lithistid the Desmacidine extended its growth inwards, coating the 
Lithistid skeleton with its own choanosome for a distance of about 2 mm. inwards from 
the surface (PI. XXXIL fig. 2). The small portion of the Lithistid which remained alive 
in the centre of the sponge was examined by means of thin slices. It consists of sarcen- 
chyma, which stains very faintly with hmmatoxylin, traversed by small excurrent and 
incurrent canals, which communicate with flagellated chambers in a diplodal fashion 
(PI. XXXIL fig. 3). The chambers measure 0*018 mm. in breadth, and 0*015 mm. in 
length. That this tissue does not belong to the Desmacidine is evident from its 
character, since in the latter the flagellated chambers are eurypylous and the mesoderm 
collenchymatous ; that it does belong to the Discodermia is conclusively proved by 
the occurrence of microstrongyles and microxeas within it. 
The oscules are beyond the reach of observation, the Desmacidine having completely 
covered them over. The pores are well shown on the surface of the exposed basal 
patch : they are single apertures bounded by ectosomal discotrisenes and phyllotrisenes, and 
lined by epithelium and its associated microstrongles. They are oval or circular in out- 
line, and from 0*045 to 0*065 mm. in diameter. 
