ii. hid 
STUDIES IN SPICULE FORMATION. 239 
narrow stalk, on the other hand, the triradiates are regu- 
lar). ‘ 
The quadriradiate spicules each consist of a triradiate basis, 
bearing on its gastral aspect an additional ray, which is 
apposed at the common junction of the three ray axes (or 
near it), and at right angles to them. These additional rays, 
so attached to a certain proportion of the triradiate spicules, 
are fairly uniformly distributed over the interior surface of 
the gastral cavity of the sponge, and project into this cavity 
with a slight upward inclination. It is not improbable that 
this upward inclination is directly due to the current of water 
which constantly flows up the gastral cavity and out of the 
oscular aperture. The gastral actinoblasts (formative cells) 
which deposit the substance of these gastral rays, as they are 
termed, must be influenced by such a current, and the in- 
clination of the rays is perhaps the visible expression of this 
influence; at least there is no other assignable cause. The 
gastral rays probably possess no function, being, as will here- 
after be explained, inevitable results of the architecture of 
the sponge wall. 
The mode of formation of these three forms of spicule will 
be described first: theoretical problems being reserved for 
separate consideration in Part II. 
As a preliminary to the following description, it is well to 
mention, especially in view of Maas’ misinterpretations, that 
it 1s sometimes necessary to use a certain amount of dis- 
1 In connection with the ‘tuning-fork” triradiates of Clathrina lacu- 
nosa developed in the thin swaying stalk of that species, it may be pointed 
out that (as shown in text-fig. 1) even in Sycons in which the cylindrical 
body is of much wider diameter, the paired rays of the majority of the tri- 
radiates certainly enclose Jess than 120°. ‘Ihis conformation is doubtless an 
approach towards the “ tuning-fork ” type of spicule, a type always produced 
in connection with the swaying of a narrow cylindrical sponge-body. It will 
be seen from this (and still better from what follows in Part IJ) that the 
movements of the sponge-wall can influence the triradiate spicule in two ways: 
flexion of the curved wall without invagination tends to render the paired 
rays more vertical in inciination (see also p. 268 for the cause of the vertical 
disposition of the monaxons), whereas invagination tends to depress them. 
