STUDIES IN SPICULE FORMATION. 301 
the spicule as unavoidably secreting lime—the cell “ cannot 
help it ”’—and the faster it secretes the better is it nourished, 
and therefore able to secrete, since additional secretion of 
lime implies further increase of surface area, and this (although 
the scleroblast is not a plant) is favourable to the nourishment 
of the cytoplasm. However, as there is a limit to the divisi- 
bility of a Paramecium, however well nourished,! so there is 
a limit to the growth, and therefore secretory power of the 
enormously-distended scleroblast with its two nuclei, which, 
judging from appearances in some of the full-sized spicules I 
have seen, like the Paramecium, eventually degenerates. 
The massiveness and irregularity of the majority of the 
spicules in Aleyonium digitatumis due to the fact already 
suggested that all older spicules are situated in mesogleeal 
substance far removed from any limiting layer, the colony of 
A. digitatum being, as is well known, exceedingly massive 
in form and not racemose. When, however, spicules have 
existed for the greater part of their development in close 
proximity to the external ectoderm or to the endodermal 
layer of a gastral canal, irregularity of form largely disappears 
and the spicules assume the form of a simple manaxon either 
with very minute processes or without any (fig. 15). As 
Hickson says, “there may be seen a certain number of long 
unbranched lancet-shaped spicules covered with irregular 
tuberosities. They vary very much in shape, some being like 
a thick pin (without its head) others lancet- or spindle-shaped, 
and others again slightly curved like a boomerang. ‘These 
long unbranched spicules occur chiefly in the tentacles and 
disc of the polyps [where the mesoglcea is evidently of narrow 
width]. They do occur in other parts of the colony [in 
proximity to the endoderm canals], but I do not remember to 
have seen any dumb-bell-shaped spicules in the extensible 
portion of the polyps.” Again, in the other English species, 
Aleyonium glomeratum, which is much more racemose 
in form than A. digitatum, and in which consequently the 
1 In view of Calkin’s experiments, I suppose I ought to add “on one 
diet,” 
