ACANTHASCIN^. 10/ 



in a straight line in sncli a manner that the point of junction 

 of the two parls is externally not in the least indicated ; but the 

 terminal may sometimes be l)ent at base exactly as in a hexaster 

 ray with two divei'gent terminals, save that one of these terminals 

 is entirely -wanting, in which case it may be said to be well 

 marked off from the principal bearing it. 



The discoctasters are spicules peculiar to the Acanthascinae ; 

 in fact their presence constitutes the only reliable criterion by 

 Avhich a Kossellid can be determined as a member of that 

 subfamily. Hence a misgiving might be entertained that should 

 they chance to l)e simply overlooked or not properly identified, 

 or if there existed a species which had lost them only secondarily, 

 the sponge in question would likely be taken up under the 

 Hossellinse, which are apparently ab origine without these charac- 

 teristic spicules. 



The discoctasters were first reco2;nized l3v F. E. Schulze 

 ('93) to l)e strongly modified discohexasters in which the six 

 ])rincipals have entirely or almost entirely atrophied while the 

 terminals have undergone a new arrangement into eight secondary 

 principals and terminal tufts at points of the central node cor- 

 responding to the eight corners of a cube. It was pointed out 

 l)y the above-mentioned writer that there may exist on the central 

 node and in the center of the space surrounded by every four 

 secondary principals a hump-like prominence representing the 

 outer end of a primary princij^al, and further that this protuberance 

 may run out at base into four radial ridges (see PL XL, fig. 20), 

 marking the course along which the original terminals were laid 

 down in order to combine into the eight secondary j^rincipals. 

 These are frequently not quite cylindi'ical being longitudinally 

 ribbed or at any rate somewhat angulate in cross-section, indicating 



