194. MARINE BIOLOGY OF THE SUDANESE RED SEA. 
the flagellated chambers, occurs at the base of the collar in Tenthrenodes 
antarcticus.” (Jenkin, 9.) 
We thus have a description of the formation of chiacts direc’ from gastral 
quadriradiates by a direct change of position. The result of the “tipping” 
in Tenthrenodes is less clear, as there never occur any subdermal spicules in 
this genus, 7. e. spicules with a ray pointing gastralwards. 
As regards the two species of Grantzlla, I have been unable to make any 
investigations on G. quadriradiata, as the single specimen obtained by 
Mr. Crossland consists of a fragment from which the oscular portion has been 
lost. In G. hastifera I found that neither subgastral nor subdermal triradiates 
or prochiacts extend quite to the oscular rim, but that there is, as in the 
Chiphorid and Staurorrhaphid species, a small area in which they are not 
arranged absolutely radially, but are slightly inclined on both surfaces. This 
inclination does not seem to be always in the same direction, but may be either 
oral- or aboralwards. The spicules in this region of the sponge are considerably 
smaller than the full-grown ones, being not much larger than the triradiates 
of the dermal and gastral cortices. Above the region of irregularly placed 
prochiacts and triradiates it was found impossible to distinguish them from 
the cortical spicules. Owing to the preservation of the specimens not being 
sufficiently good, I found it impossible to obtain any histological details of 
their development. 
It is therefore impossible to be absolutely certain that these subdermal and 
subgastral spicules are derived directly from the dermal and gastral cortical 
triradiates by a change of position, though the facts given above and the great 
similarity of the spicules in the different genera are very strongly in favour of 
such a derivation; and it seems undoubtedly easier to so derive them, especially 
as it is possible to do so in entire harmony with the known facts, rather than 
to consider them an entirely new structure with an origin entirely independent 
of any existing spicule. 
The retention of the paired rays of the prochiact in the primitive facial 
plane can also be explained very easily in accordance with this view. The 
change of position of the spicule must undoubtedly commence while it is quite 
small, and must be a gradual process, as it presumably depends upon the 
growth of the flagellated chambers under it for its accomplishment, at any 
rate in the case of the subgastral spicules. 
Since the gastral and dermal cortices are separated by the growth of the 
flagellated chambers, there will be no obstruction in the way of the basal ray 
as it gradually assumes the radial position ; but to the movement of the paired 
rays, which it must be remembered are growing during the whole of this 
period, a very serious obstruction is opposed by the presence of a cortex. 
Were the additional calcite secreted by the calcoblasts deposited along the 
line of the axis of the ray, the ray as it elongated would have to force its way 
through the cortex as the spicule turned, whereas this difficulty would be 
