REPORT ON THE TETRACTINELLIDA. 
297 
one another by overlapping, some on the contrary join by a true articulation, the edge of 
one being received between thickened outgrowths from the upper and under surfaces of 
its opponent (PL XXXII. fig, 19). 
Though the rhabdome usually preserves its spicular character, yet there are instances 
in which it shares the fate of the cladi, either growing out into an expanded plate, and 
then exchanging the usual position normal to the surface for one of parallelism with it ; 
or assuming other irregular forms and positions, as in fig. 18, where it has become bent 
up into the same plane as the disc, and curved and thickened to form what appears to 
be a small poral tube. 
It is then by the adaptation of the cladomes to one another, and to the pores, that 
their sinuous outlines may be explained, and not on the assumption that they are passing 
into true desmas of the interior skeleton. Yet it is to be especially observed that the 
discotrisenes differ from true spicules in much the same way as the desmas of the interior; 
at least this is true of the cladome, but not usually of the rhabdome. The axial rods 
of the cladi of the trisene spicule from which they arise cease to direct the formation of 
the cladome after a very early stage, just as the axial rods of the desma cease to 
influence the growth of the epactines ; the cladomes tlien exhibit subsequently just the 
same power of adaptability as the desma, even to articulating together in a somewhat 
similar fashion. 
Discodermia ornata, n. sp. (PI. XXXI. figs. 1-6). 
Sponge (PI. XXXI. fig. 1). — Vase-shaped, with a short stout pedicel, expanding to an 
attached incrusting base ; margins of the cup thin, rounded. The outer surface of the 
skeleton presents the open circular mouths of incurrent canals, irregularly dispersed, and 
shallow tubular grooves, which wander from the mouth of one incurrent canal to that of 
another, forming a superficial network ; these grooves are the exposed subdermal cavities. 
The inner surface presents the mouths of the excurrent canals irregularly dispersed, 
they are largest and most closely clustered together at the base, where the vertical canals 
of the stalk open. 
Spicules. — I. Megascleres. 1. Desma (PI. XXXI. figs. 6,6a-6c), usually of very regular 
tetrad form, with simple, or once or twice branched, cylindrical epactines, smooth, or covered 
with tubercles, which may be simply conical with rounded ends, or cylindrical with a 
flat summit and rounded edges, or cylindrical and dividing at the top into two or more 
smaller rounded or hemispherical tubercles. The ends of the cladi expand into tubercles, 
which adapt themselves to the ends or sides of neighbouring branches. The average 
length of the simple epactine is from 0T3 to 0'26 mm., of the epactine of cladose forms 
from 0T3 to 0'2 mm., the average diameter varies from 0'05 to 0’7 mm. The length of 
the axial rod differs within comparatively narrow limits from 0'052 to 0’071 mm. 
(ZOOL. CHALL. EXP. — PART LXIII. 1887.) ElT’ 38 
