STUDIES IN SPICULE FORMA'ION. 247 
bases, begin to travel up their respective rays, so following 
in the course of the apical cell (figs. 26—30), and, like this, 
ultimately deserting the spicule. It is possible that during 
its migration towards the extremity of the ray, the basal cell 
may supply a thin secondary coating of lime, but this I have 
never observed. The destination of the apical and basal 
cells after leaving the spicule is unknown, and I have not 
attempted to ascertain it. 
It is worth while remarking that after the apical cell has 
separated from the basal cell, the two cells apparently carry 
on their work quite independently of each other, and, as we 
have seen, their respective functions are, after the initial 
deposits have been formed, essentially distinct in nature. 
This subsequent independence of activity, or, otherwise 
speaking, lack of co-operation, is well shown by a curious 
type of spicule found by Minchin? in Leucosolenia com- 
plicata which he has termed “derelict.” In this type of 
spicule, whilst the basal cells have been fully active, the 
apical cells seem to have shirked their duty, and the result is 
a large nodular mass with three small irregular rays arising 
from it—a conformation not only clearly exhibiting the 
independence, but also the respective natures of the activities 
of the two sets of cells. 
THE QUADRIRADIATE SPICULE. 
As before stated, quadriradiate spicules are simply tri- 
radiates plus an additional ray, which is attached to the 
common junction (or near it) of the triradiate basis on its 
gastral aspect, and at right angles to the plane in which 
the triradiate lies—the plane of the sponge wall. The 
derivation of the mother-cell of this additional ray—the 
gastral ray—I have not been able to discover as yet, owing 
to the complexity of structure of the body-wall introduced by 
1 The non-cylindrical disposition of the substance of the basal cell (see 
p. 250) may be worth remarking in this connection. 
2 Prof. Minchin has not yet published these researches, but will shortly 
do so. 
vou. 49, PART 2,—NEW SERIES, 18 
