10 PROF, E. A. MINCHIN ON THE [May 2, 
of view of substantive variations of form and size; secondly, as 
regards numerical variation, that is to say abundance of monaxons 
compared with other types of spicule. 
The monaxons are all of large size, being at least twice as 
thick as the basal rays of the triradiate systems, and not less than 
300 » in length, allowing for those which are apparently not full- 
grown. But in some specimens the monaxons reach a size which 
can only be called gigantic. In a specimen from Banyuls sent 
me by Topsent (which I will refer to as Topsent 12¢), the 
monaxons, when drawn to the same scale as the other spicules 
figured here, come out 32 centimetres in length, corresponding to 
an actual length exceeding 1000p (1 mm.), with a breadth of 
about 50 at the thickest part. Even these proportions are 
exceeded by a specimen in my collection from Banyuls, in which | 
the monaxons when drawn to scale measure 75 centimetres in 
length, corresponding to an actual length of 2343  (2°3 mm.). 
I do not think that spicules of such size have been recorded from 
any Ascon. The large monaxons of Ascandra densa and A. parus 
figured by Haeckel (J. c. pl. 14. figg. 2c, 3,f) fall far below those 
that I have mentioned in dimensions. With these extraordinary 
variations in size, the form and characters of the monaxons are 
fairly constant (text-figg. 2 and 3, 1j-lm, 2h, 3c-3h). They 
are spindle-shaped, pointed at both ends, slightly curved, some- 
times distinctly so when more slender, or nearly straight when 
very thick. There is no lancet-head present at the distal ex- 
tremity, as figured by Haeckel; his figure (/.c. pl. 14. figg. 6 d, 
6 ¢) almost certainly refers to complicata (compare his figg. 1 g—-1 k, 
on pl. 15). It is, indeed, impossible to say which is the distal end 
of these monaxons, as they do not project from the sponge like 
the true (primary) monaxons of other Ascons. Near the middle of 
the spicule, sometimes at about one-third of the length from one 
end, a slight constriction can be observed, sometimes very distinct, 
in others very shallow, in others again represented by an annular 
thickening, and sometimes not to be made out at all. This con- 
striction is more distinct in young spicules, and appears to become 
more or less obliterated with growth. In big spicules the 
contours are often so sinuous and irregular that the primary 
constriction may be masked by secondary curves. I consider this 
primary constriction, as I propose to call it, of great morphological 
importance, as indicating probably that these spicules are not 
primary monaxons*, comparable to those of  Leucosolenia 
conyplicata, for example, but in reality derived from a triradiate 
by loss of one ray and shifting of the two others into approxi- 
mately the same straight line. In very young monaxons of 
contorta I have noticed a delicate transverse line in the region of 
the constriction (text-fig. 3, 3¢), and I have also found a spicule 
of which it would be difficult to affirm whether it is a young 
* A primary monaxon is derived from a single mother cell which divides into two 
formative cells, thus originating in exactly the same manner asa single ray of a 
trivadiate system. 
