25 
Species of Echinonematous Sponge. 
8 . The cell gives off three buds from its distal face, which 
grow outwards away from the sponge, and a fourth from its 
proximal face, which grows inwards, and we have the forked 
forms of Geodia and the like (fig. 3, 9). 
9. The cell grows in five directions along three axes at 
right angles to each other, which are not determinately related 
to any lines of reference within the sponge ( Dercitus Buck- 
landi ), or which are so related ( Euplectella and other Hexac- 
tinellids), and we have the quinqueradiate form (fig. 3, 12 ). 
10. In fig. 3, 6, the growth of the three rays is along direc¬ 
tions inclined somewhere about 120° with each other; if two 
of the rays grow in opposite directions, and the third at right 
angles to them, fig. 3, 10 , results (a form abnormal in Plectro- 
nella , frequent among the Hexactinellidas). 
Fig. 3, 11 , requires no comment. 
Fig. 3, 13, is the result of a sexradiate growth of the cell 
along three axes at right angles to each other, and represents 
the typical Hexactinellid spicule. 
Fig. 3 , 14, is an octoradiate form, seven buds having grown 
out radiately in one plane and the eighth at right angles to 
them • it occurs in the fossil Hyalostelia. 
The foregoing remarks have arisen out of our description of 
Plectronella papillosa , which was the main object of this 
paper ; but the variability of sponge-spicules is far too impor¬ 
tant a subject to be treated thus incidentally, and might fur¬ 
nish material enough for a lengthy memoir. No sponge that 
has come under my observation has failed to exhibit numbers 
of spicules departing more or less widely from the average 
type ; frequently the range of variability is extreme; and no 
doubt, when a large number of specimens of allied species of 
sponges come to be carefully compared, we shall find not only 
in their external form, but in the details of their internal struc¬ 
ture as w'ell, easy passages from one to the other, and links 
will be discovered uniting together types of sponge-structure 
that now appear widely separated from one another. 
Taxonomical position of Plectronella.—The existence of 
distinct fibres echinated by characteristic spicules places this 
sponge in Carter’s fifth order, the Echinonemata. 
In the general structure and arrangement of its fibres and 
the position of its echinating spicules it most resembles the 
genus Dictyocylindrus (Bwk.). 
In the form of its echinating spicules it appears to approach 
the crutch-shaped spicules of the group Baculifera (Carter), 
founded on Savile Kent’s Caulospongia. In the Baculifera, 
however, the crutch-shaped spicules form the core as well as 
echinate the surface of the fibre, while it is only exceptionally 
