a neio Oenus and Species of Monaxonid Sponges. 157 



though some of these larger forms still present a merely 

 rounded end, others possess in addition from one to three 

 short conical teeth budded off at some little distance before 

 the end. There is still not the slightest trace of anj ter- 

 minal inflation, such as occurs in T(?<«7/a-grapnels. The 

 rays arise merely as spines, precisely similar at this stage to 

 the more numerous spines which cover the distal end of the 

 quadriradiate spicules of Tricentrium muricatuin. We may 

 indeed, on the basis of these observations, regard the rays of 

 these grapnels as highly developed spines, which, at their 

 inception indefinite in number, become subsequently limited 

 to three." 



Leaving now the embryological evidence, which clearly 

 demonstrates that the tetractinellid spicule is derived from 

 the monaxonid form, we must turn to the evidence afforded 

 by several species usually recognized as Monaxonid sponges. 

 First of all it must be observed that the actual number of rtiys 

 to the spicules in Tetractinellid sponges is by no means con- 

 stant. Prof. Sollas has very kindly supplied us with infor- 

 mation on this point : he tells us, " the variability in number 

 of the teeth is a matter of no moment, so long as they do not 

 frequently exceed three. Most Tetillas exhibit extreme vari- 

 ability in this respect, the same sponge frequently presenting 

 forks or anchors with one, two, or three teeth." 



We have already referred to the evidence afforded by the 

 Suberitid sponge Polymastia [Radiella) schoenus. 



Of other Monaxonid sponges with polyaxial skeleton-spi- 

 cules the genus Acarnus forms a very good example. Both 

 the known species of this genus possess grapnel-spicules 

 echinating the skeleton-fibre in tufts. Each spicule has a 

 rounded base attached to the skeleton-fibre, a straight smooth 

 shaft, and a grapnel-like apex ; in the one species the number 

 of teeth in the grapnel is four {A. tnnominatus), and in the 

 other three {A, tematus). It is of great importance to notice 

 the position of the grapnel-spicules in this sponge — that they 

 occur within the body and not, as in the Tetractinellida, mainly 

 radiating from the surface ; in the one case there is a radiate 

 arrangement, and in the other there is none ; hence, though 

 Acarnus tematus possesses grapnel-spicules whose well-deve- 

 loped teeth are almost constantly three in number, yet, 

 having regard to other features, such as the arrangement of 

 the skeleton, it will be seen to come not nearly so close to the 

 Tetractinellida as does Proteleia. 



In anotiier very interesting monaxonid sponge obtained by 

 the ' Challenger ' and to be described fully in our report, under 

 the name Thrinacophora funiformisj there is, amongst other 



