78 
THE VOYAGE OF H.M.S. CHALLENGER. 
II. Microscleres. 5. Plesiasters of the usual form ; single actine of a tetrad form, 
0’079 by 0’004 mm. ^ 
6. Metasters, spines about 0’019 mm. long. 
7. Spirasters, a slender spire with comparatively few long, slender, pointed spines ; 
spire 0'012 mm. long, spines 0’012 mm. long, total length 0’032 mm. 
Colour. — Brownish-grey. 
Habitat. — Station 297, off west coast of South America, November 11, 1875; lat. 
37° 29' S., long, 83° 7' W. ; depth, 1775 fathoms; bottom, Globigerina ooze; bottom 
temperature, 3 5° ’5. 
RemarTcs. — Only a single specimen of this sponge was trawled ; it is small but 
perfect, 4 or 5 mm. in height, and 3 mm. in diameter, excluding the projecting spicules. 
It is probably a distinct species, but as from its small size it may be supposed to be 
immature, I refrain from giving it a separate name. The main excurrent canals descend 
vertically from the cribriform area at the summit ; they are provided with thick collen- 
chymatous walls, constricted at intervals by velar diaphragms, and riddled with lateral 
openings leading into branch canals. The ectosome is about 0'02 to 0'04 mm. thick. 
The greater part of the choanosome is occupied by the flagellated chambers, which in their 
expanded state measure as much as 0’055 mm. when spherical, and 0‘059 by 0‘043 mm. 
when oval in section. Many of them are contracted, and then exhibit the backward 
extension of the body of the choanocytes into a long filament which has been described 
previously {vide, Thenea delicata, p. 63, PL YI. fig. 20). 
The specimen differs from other species of Thenea in the absence of a special 
poriferous area, but this is probably a mark of immaturity ; in the presence of a basal 
fringe of hispidating spicules, which appear to take the place of rootlets ; and in the 
presence of numerous protrisenes, which are rare in other species of the genus. The 
presence of an additional character (that of the basal fringe), coinciding with the absence 
of a common character (that of a special poriferous area), led me to think that what I took 
for the base might really be the special poriferous area, and the basal fringe the spicules 
surrounding it, I therefore searched for some signs of a poriferous area in this position in 
my thin slices, but could not detect any. It seems very possible that this may be the 
young form of Thenea wrightii. 
Thenea sp. 
A small and fragmentary sponge, evidently not adult ; the spicules are similar to 
those of Thenea muricata. As it is without characteristic features, I think it sufficient 
to record its occurrence. It may possibly be a young form of Thenea wyvilli. 
Habitat. — Station 218, near the Admiralty Islands, March 1, 1875 ; lat. 2° 33' S., 
long. 144° 4' E. ; depth, 1070 fathoms ; bottom, blue mud ; bottom temperature, 36°’4. 
