24.0 W. WOODLAND. 
crimination in describing the spicule and the cells in connec- 
tion with it. Without due care, it is at first possible to err 
in distinguishing between the various adjacent cells—epi- 
thelial cells, collar-cells, pore-cells, wandering-cells, and 
isolated future spicule-cells (scleroblasts), not to name cells 
belonging to other spicules than the one under observation— 
and the formative cells of a particular monaxon or triradiate, 
but with a little practice such mistakes cannot be made. 
Criteria of those cells which alone appertain to an individual 
spicule are as follows :—(a) position of cell—the cell always 
being in close proximity to the ray (especially to be observed 
in regard to the vertical, i.e. to the focus) and (in viewing 
triradiates from the gastral surface) with the nucleus lying 
in the plane of, or below, the spicule if basal in position (i. e. 
central), and above if apical (i.e. at end of ray); (b) form of 
cell—such, as will be seen from the figures provided, always 
having a well-marked relationship to the ray, as regards the 
long axis of the cell and the cell-contour; (c) character of 
cell—scleroblasts inter alia possessing greater detiniteness 
of form than epithelial cells, a larger nucleus and less granu- 
lation than the choanocytes, difference of outline from pore- 
cells, and more granulation and absence of refringency as 
regards amcebocytes. 
Unless otherwise stated, the following account refers to 
Sycon coronata; however, a like version holds true for 
S. ciliata, save in certain minor particulars which are duly 
considered in their place. 
THe Monaxon SPICULE. 
In both the species of Sycon examined the monaxons vary 
considerably in thickness, as the accompanying text-figure 
shows; nevertheless, the process of deposition is the same for 
all, though it is possible, and even probable, that the large 
thick monaxons have a slightly different origin from that of 
the thin variety (Appendix B). Judging from the few 
