STUDIES IN SPICULE FORMATION. 543 
of the same species, there is every reason to believe that their 
development is of the same type. The knobbed spicules very 
much resemble the common variety as regards approximate 
regularity of growth, but differ as regards their thickness, 
the small size of their perforations, and the development of 
knobs. Knobbed spicules containing four apertures are 
usually found with either two or four scleroblasts adhering 
to them, and indeed the stages depicted in figs. 17—19 
are extremely common. They and older stages are usually 
found situated on the interior side of the wall with spicules 
of the (c) variety; as usual, the most massive spicules are 
situated internally. The reason for the development of 
knobs on these particular spicules I am quite unable to 
ascertain. 
Of spicules of the (c) variety I have found several fairly 
young stages (figs. 20—24), from which it will be observed 
that the two types of development—the bi- and tetra-sclero- 
blastic—also obtain here (cf. figs. 20—22). Another feature 
concerning this type of spicule is the small number of sclero- 
blasts associated with the larger plates (figs. 283—30). Also 
most of the adult forms of these spicules are more or less 
elongated, and they he with the long axis parallel with 
that of the animal. 
In the other species, C. brunnea, the plate spicules are 
larger than any to be found in C. sp., but at the same 
time are fewer in number. Initially the development is as 
usual of two types; at least, judging from figs. 31 and 33, 
it is. Like the variety (c) of C. sp., the number of 
scleroblasts associated with the older spicules is small, and 
the plates (only the very young are shown in the figures) are 
elongated. The scleroblasts are very plentifully supplied 
with the dark granules. 
With regard to the small superficial spicules common to 
both ©. sp. and C. brunnea, I have been least suc- 
cessful in ascertaining their mode of origin and develop- 
ment owing to difficulties of observation.! Apparently they 
' See further in section on Thyone. 
