REPORT Olf THE TETRACTIHELLIDA. 
15i> 
specimen was obtained in Agassiz’s expedition of 1879 off Morrolight in 250 to 400 
fathoms. 
Remarks . — Of this singular and interesting sponge, only the cloacal tube was known 
to Weltner, who not unnaturally regarded it as a complete organism, although a histo- 
logical examination would have revealed the absence of choanocytes and flagellated 
chambers, and thus have suggested its real nature. The specimens of the Challenger 
collection are fortunately entire, and thus afford an opportunity for complete exam- 
ination and full description. A longitudinal section through the sponge (PI. XVII. 
figs. 9, 10) reveals a cortex sharply differentiated from the choanosome, which is 
restricted to the spherical body, and does not extend into the cloacal %tube. This latter 
commences in a pointed extremity at the centre of the sponge, where also the spicular 
fibres of the skeleton originate, thence it gradually enlarges as it proceeds radially out- 
wards, so that within the sponge-body it has the form of an inverted cone, with its base 
at the periphery and its apex at the centre of the sponge. It is continued outside the 
sponge-body as an almost cylindrical tube, tapering very gradually towards its distal 
termination, which is truncated, and closed by a sphinctrate diaphragm, centrally per- 
forated by the terminal oscule. The tube is usually slightly curved, with a tendency to 
be twisted. The following are the dimensions of six of the most perfect specimens in the 
collection: — (1) spherical body (spongomere) 20 mm. in diameter, cloacal tube, conical basal 
part 10 mm. long, diameter on emerging from the spongomere 8 mm., length to which 
it extends beyond the spongomere 65 mm.; (2) spongomere 16 mm. in diameter, cloaca! 
tube 63 mm. long, 6 mm. wide at its point of emergence, 2 mm. wide at its distal 
termination; (3) spongomere 12 mm. in diameter, cloacal tube 4 mm. wide at its point of 
emergence, 45 mm. in length; (4) spongomere 11 mm. in diameter, cloacal tube 45 mm. 
in length; (5) spongomere 8 mm. in diameter, cloaca] tube 4 mm. wide at its emergence 
from the spongomere, 32 mm. in length; (6) spongomere 6 mm. in diameter, cloacal tube 
6 mm. long. Besides entire sponges there are a few separate cloacal tubes in the 
collection ; one of these measures 95 mm. in length. 
S'pongomere — Ectosome. — The cortex is of irregular thickness : in a specimen 8 mm. 
in diameter it varies from about 0‘22 to 0'39 mm. in thickness ; it consists of a middle 
collenchymatous layer about OT to 0’2 mm. thick, faced externally by a thin fibrous 
layer about 0'002 mm. thick, in which the constituent fusiform cells run parallel to the 
surface, and continued inwards into a thick layer of fibrous tissue, from about OT to 0'2 
mm. thick, in which the fusiform cells arranged in fibres run in various directions, 
mostly tangentially. The collenchymatous layer, which is widely excavated by inter- 
cortical canals, contains, as weU as the usual collencytes, numerous fusiform cells, which 
cross obliquely from the outer to the inner fibrous layer, surround more or less 
concentrically the intercortical canals, and wander in various other directions; some 
