RHABDOCALYPTUS MOLLIS. 265 



The subdermal oxyhexasters (PL XX., fig. 3) are distinguished 

 by being nearly always of the normal shape and by having very 

 slender terminals. Each exceedingly short principal in a spicule 

 carries 2-4 (usually 3), nearly straight, rough terminals. These 

 seem to be very fragile ; the broken off terminals are usually 

 found in numbers sticking to the dermal membrane. The rough- 

 ness of surface becomes more pronounced towards the base of the 

 terminals, where it may distinctly appear to be due to reverted 

 microtuberclcs. Under certain circumstances these may be de- 

 veloped even into long and conspicuous barbs, but such is by no 

 means the case with the generality of the peripheral oxyhexasters. 

 Karely is the variety in question hemihexactinose or hexactinose. 

 Deep in the choanosome it is only occasionally met with pro- 

 miscuousl}^ with the following. 



The second variety (PI. XX., figs. 0-11) is by far the more 

 numerously represented, occurring in greatest abundance in the 

 subgastral region as well as in the endosomal layer. In it the 

 principals are reduced almost to nothing, so that the terminals 

 appear to radiate directl}^ from the central node. The terminals 

 are — in some individuals of the sponge slightly and in others 

 decidedly — stronger than in the other variety. In some exceptional 

 cases of the spicule I have found all the rays nearly^or quite 

 smooth from tip to base ; but the general rule is that their basal 

 parts show^ a varying number of barbs or unusually strongly 

 developed retroverted microtuberclcs, while the distal parts are 

 either obsoletely rough or quite smooth. Sometimes the basal 

 barbs occur in quite a limited number ; it may even happen that 

 in the same spicule some rays are provided with only a few of 

 them while the rest are entirely smooth. As a general rule, 

 however, the b;irbs are numerous, gradually increasing in length 



