346 BE. A. MINCHIN. 
factor in controlling the production of forms of spicules, 
while in Demospongie adaptive influences are allowed exclu- 
sive sway. It is a natural tendency of the mind to seek for 
uniform explanations in parallel cases. But if we accept 
Schulze’s three main stems, or rather branches, of sponge- 
ancestry, it is probable, indeed, almost certain, that in each 
stem the spicular skeleton was acquired independently, and 
might therefore have been subject to different controlling 
influences in each case. 
With regard to the evolution of the spicules within the 
group of the calcareous sponges I have but little to add to 
the conclusions I put forth in 1898 (p. 568). Ithen regarded 
the triradiate system as a fusion of three primitively separate 
monaxon sclerites, and I am more than ever convinced of this, 
now that [ have found that the primary monaxons develop in a 
manner perfectly similar in every way to a single ray of a 
triradiate. I regarded, and still regard, the ancestral form 
of spicule in Calcarea as “a simple monaxon placed tangenti- 
ally, and completely embedded, in the body-wall,” and I 
believe that the typical monaxon of Calcarea, as now occur- 
ring, for instance, in Leucosolenia, arose from the primitive 
type by a process of accretion at one end, causing the older 
portion of the spicule to protrude from the surface of the 
body as the distal projecting extremity. ‘The triradiates, on 
the other hand, arose by fusion of three primitive monaxons ; 
and as I have pointed out above, the fusion to form tri- 
radiates took place by that extremity of the sclerite which, 
in existing monaxons, projects from the surface of the body. 
From von Ebner’s results, with regard to the regular tri- 
radiates, | think we may add a further conclusion, namely, 
that the primitive, tangential monaxon had its crystalline 
optic axis at right angles to its morphological longitudinal 
axis; in fact, that the optic axis of the ancestral monaxon 
sclerite was vertical to the body-wall, that is to say, radial to 
the longitudinal axis of the primitive Olynthus. The pro- 
jecting monaxons of existing Calcarea have, as von Ebner 
has shown, the morphological axis inclined to the optic axis, 
