74 
THE VOYAGE OF H.M.S. CHALLEHGEE. 
0'25 mm. across ; it undergoes a considerable thickening at the margin of the poriferous 
recesses, and is continued inwards as a wall to the incurrent canals. The choanosome is 
amply supplied with collenchyme, which forms a wall to the main excurrent canals and 
their branches, except those into which the flagellated chambers open. The chambers 
are fairly large, 0'06 by 0‘04 to 0'065 by 0'055 mm. The plesiasters occur chiefly in 
the walls of the canals, where they are so situated that two or more of their rays extend 
parallel to the surface of the canal, and one projects from it at right angles ; so that the 
cavity of the canal is echinated throughout. 
The spiraster occurs in the choanosome as well as the ectosome ; it varies in 
dimensions, especially in the length of its spines. 
Thenea wyinllii, Sollas (PL YI. flgs. 1-9). 
Tlienea wyvilUi, Sollas, Prelim. Account, Sci. Proc. Eoy. Dubl. Soc., vol. v. p. 184, 1886. 
Sponge (PI. VI. flgs. 1, 2). — Upper surface rounded, cushion-like or flat; equatorial 
margin sharp, thin, and not provided with a spicular fringe ; projecting considerably 
over the lower surface, which is produced into large conical processes from which several 
strong roots are continued downwards as short flbres, which splay out at their lower ends 
and lose themselves in a dense mass of tangled spicules forming a basal support. The 
oscule in the centre of the upper surface is the open mouth of a wide, somewhat shallow 
basin-like depression or cloaca, which is lined by a delicate, smooth, minutely perforate 
membrane, below which the numerous excurrent canals open by small circular mouths, 
arranged more or less in vertical or radial series. The minute perforations of the cloaca! 
membrane are sometimes replaced — either along the oscular edge or generally — by large 
oval fenestrse, radially elongated and serially arranged, within which the excurrent canals 
open, one or more to each fenestra, by a sphinctrate aperture. The equatorial poriferous 
membrane, of which the pores are unusually small, is very finely, closely, and regularly 
striated, owing to the close and regular disposition of its supporting fibrous strands. 
Although the equatorial membrane is continuous round the sponge, the equatorial 
sinus is broken up into a number of subsidiary cavities by the frequent attachment of the 
membrane to the underlying tissue. 
The incurrent and excurrent canals are lined by a thick layer of collenchyme. 
Flagellated chambers eurypylous, from 0‘032 to 0'04 by O'OSl mm. in diameter. 
Spicules. — I. Megascleres. 1. Oxea, fusiform, usually curved, sharply pointed, 7 ‘8 5 
by 0‘07 mm. to 0‘084 mm. 
2. Protrisene, with short conical cladi, straight or slightly undulating ; sometimes 
reduced to two or even one in number; rhabdome 6‘783 by 0'072 mm. ; cladi 0’5 mm. 
long. 
3. Dichotrisene (PI. VI. fig. 4) of the usual form, except as regards the rhabdome. 
