MATERIALS FOR A MONOGRAPH OP THE ASCONS. 545 



The last of these possibilities he sets aside at once, as 

 entirely improbable and unsupported by facts. The second is 

 regarded as undoubtedly true in some cases ; but the first is 

 the true relationship between monaxons and triradiates in 

 general. The vast majority of monaxon spicules in Calcarea 

 are regarded as primary forms, fundamentally distinct from the 

 other primary form, the regular triradiate spicule. There are 

 thus two stems or lines of descent marked out in Calcarea, the 

 first starting from the genus Ascetta, the second from the 

 genus Ascyssa. The two primary forms of spicule are traced 

 back by Haeckel to two distinct types of biocrystal. ''The 

 fundamental form of all monaxon spicules is the absolutely 

 regular spindle, or a cylinder, on the basal surfaces of which 

 are seated two similar cones with curved mantle surfaces. The 

 original and fundamental form of all triradiates and quadri- 

 radiates is . . . the absolutely regular triradiate, which can be 

 considered as a hemiaxon form of the hexagonal crystalline 

 system, in which calcium carbonate crystallises as calcite " 

 (p. 377). 



Schulze, in his great work on Hexactinellids (1887, pp. 

 497 — 504), put forward a general theory of the forms of 

 spicules, not only in Calcarea, but in sponges generally. He 

 first of all attempts to trace the main lines of descent in the 

 phylum of sponges, and recognises three main stems, each 

 leading back to a distinct ancestral form, characterised by a 

 peculiar type of skeleton. The three lines of descent are (1) the 

 Calcarea, with a calcareous skeleton; (2) the Tetraxonia, with 

 siliceous spicules typically of the tetraxon type, retained in 

 the modern Tetractinellids^ and Lithistids; the Monaxonida 

 are considered as having arisen from the Tetraxonia by reduc- 

 tion of the rays of the spicule, and the Keratosa from the 

 Monaxonida by loss of the spicules and their replacement by 

 spongin ; (3) the Triaxonia, with siliceous spicules of the 

 Triaxon type, represented by the modern Hexactinellids. Each 

 of these three types must have acquired its skeleton indepen- 



^ Under the Tetractinellids, Schulze includes such forms as Plakina, 

 Corticium, &c., now usually separated in the division Carnosa. 



