86 
THE VOYAGE OE H.M.S. CHALLENGER. 
2. Orthotrisene and Calthrops of the usual characters ; actines or cladi frequently 
rounded off into short rods or tubercles ; rhabdome of trisene 0'5 mm. long; cladi 0’464 
by 0‘045 mm. 
II. Microscleres. 3. Microxea, straight or curved, sharply pointed, surface 
roughened; 0'136 by 0’005 mm. 
4. Metaster, very numerous, of every variety of form, passing into plesiasters of four 
actines, in which the actines may attain a length of 0’02 mm.; those with a straight or 
once-curved shaft, and more numerous spines spirally arranged, are about 0'0237 in total 
length, and the spines are about O’OOS mm . long. 
Colour . — Yellowish or greyish-white. 
Habitat. — Station 236, June 5, 1875; lat. 34° 58' N., long. 139° 29' E. ; depth, 
775 fathoms; bottom, green mud; bottom temperature, 37°’6. Trawled. 
Remarhs . — Three small fragments of this sponge, all from the same station, are the 
only specimens obtained. The largest fragment is 20 mm. broad by 20 mm. wide. Its 
distinction from Poecillastra laminaris rests not only on the greater thinness of its 
sponge-wall, but also on the absence both of a marginal fringe of hispid spicules and of 
spirasters. 
The canals cross the wall transversely, those of one side alternating with those of the 
other ; they are comparatively wide, but largely filled up by collenchyme, which crosses 
them in irregular vela. 
The flagellated chambers are about 0‘0237 by 0‘0316 mm. to 0’0316 by 0‘0473 mm. 
in diameter. 
The abnormal forms of the calthrops are very numerous ; it may be reduced to tri-, di-, 
or monactinose forms, the monactine or style frequently becoming spherically enlarged 
(t}dostyle) at the end from which the lost actine has disappeared ; the diactinose or 
rhabdus form is frequently swollen in the middle about the actinal origin ; on the other 
hand, additional actines may be abnormally present ; there may be five — four in one plane 
directed along two rectangular axes and the fifth at right angles to them, — or six, of which 
five may lie in one plane. These various forms resemble those of the plesiasters in 
Thenea, but while the plesiasters chiefly afford instances of reduction, the modifications 
of the calthrops are produced by increase as well as decrease in the number of actines. 
Given a spicule liable to vary, and under the action of the tensions existing in the sponge 
actines will be suppressed or developed in accordance with the tensions, and thus similar 
forms may be evolved from different starting points. 
The orthotriaenes are most abundant at the upper margin of the sponge, where 
they lie close together, with, the cladi tangential to the surface, and the rhabdome 
descending at right angles to it into the interior, in the manner usual with triaene 
spicules. 
