156 



Messrs. A. Dendy and S. O. Ridley on 



tracings of the spicule in Radiella schoenus 

 (vide annexed woodcut). The ends of 

 the chief spicule, which project beyond 

 the surface, are swollen, granulated, 

 and minutely spined. He says {loc. ci'f. 

 p. 163): — " The swollen termiiuitions of 

 the spicules of B. schoenus suggest the 

 possibility of a polyphyletic origin for the 

 Tetractinellida," 



It is important to notice that, as men- 

 tioned by Sollas, it is the projecting ends 

 of the main skeleton-spicules, and not of 

 the small cortical spicules, which in Poly- 

 mastia [Radiella) scJioen us become swollen; 

 while in Proteleia Sollasi the ends of the 

 small cortical spicule become modified. 



Evidence of very much the same kind 

 as to the mode of origin of the tetractinellid 

 spicules is afforded by the Tetractinellid 

 sponge Thenea Wallic/m, P. Wrighf^: — 

 " The slender spicules of the anchoring- 

 fibres, over which the ectoderm extends, 

 are mostly rounded at the distal end, like 

 many of the spicules of R. scliamus, or the 

 forms which so frequently occur as va- 

 rieties amongst the pin-shaped acuates. 

 These represent the first stage of the 

 grapnel-spicules, which thus differ from 

 the similar spicules in Tetilla by the 

 absence of an initiatory inflation. In the 

 next stage these spicules exhibit near 

 the distal end a number of little tuber- 

 cular excrescences, similar to those which 

 occur as abnormal thickenings on many 

 of the spicules both of the Monaxonida 

 and the TetractinellidEe. In many cases 

 these tubercles take the form of small teeth, 

 often recurved, and varying in number from 

 one to six. They are seldom situated at 

 the extreme end of the spicule, usually a 

 little distance from it. In the larger 

 specimens we find a considerable advance 

 in growth and development ; the spicules 

 show a marked increase in size : and 



* Vide Sollas, Ann. & Mag. Nat. Hist. June 

 1882, p. 450, pi. xvii. tigs. 33-42. 



Wc 







Radiella schamus, Sol- 

 las: a, chief spicule; b, 

 spicule of cortex ; c, d, 

 f, en Is of the chief spi- 

 cules which projectbe- 

 vond the surface and 

 are swollen, granida- 

 ted, and minutely 

 spined. 



