10 - PKOF. E. A. MINCHIN ON THE [^^'J 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 fj. 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 1 2 e), 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 1000 ju (1 mm.), with a breadth of 

 about 50 /x at the thickest part. Even these proportions are 

 exceeded by a specimen in my collection from Banyuls, in which 

 the monaxoias when drawn to scale measure 75 centimetres in 

 length, corresponding to an actual length of 2343 /x (2"3 mm.). 

 I do not think that spicules of such size have been recorded fi-om 

 any Ascon. The large monaxons of Ascandra densa and A. parus 

 figured by Haeckel {I. c. pi. 14. figg. 2 c, 3/) fall far below those 

 that 1 have mentioned in dimensions. With these extraordinary 

 vai'iations in size, the form and characters of the monaxons are 

 fairly constant (text-figg. 2 and 3, Ij-lin, 2 h, 3 c-3 A). They 

 ai^e 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 (Z. c. pi. 14. figg. Q d, 

 6 e) almost certainly refers to complicata (compare his figg. 1 g-l k, 

 on pi. 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 annvilar 

 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 oblitei-ated 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 

 primaiy 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 

 com2)licata, for example, but in reality derived from a triradiate 

 by loss of one i"ay 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 e), and I have also found a spicule 

 of which it Avould 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 oxactlj' the same manner as a single ray of a 

 triradiate system. 



