1908.] ON THE SPICULiES OF CALCAREOUS SPONGES. 661 



The stoppage of the growth of the ambulacrum and the plate 

 abnormalities occui-red approximately at the same time, and it is 

 suggested that they may be due to functional disturbance caused 

 by some external agent. The distortion of the test subsequent 

 to the arrested development of the ambulacrum has been brovight 

 about by a process of regulation. 



The specimen of Echinus esculentus above described has been 

 deposited in the Royal Scottish Museum, Edinburgh. 



EXPLANATION OF PLATE XXXIII. 



Fig. 1. Test of abnormal JSchinus esculentus viewed in plan, natural size. 

 Fig. 2. Test of abnormal 'Echinus esculentus viewed in elevation, natural size. 



Lettering : — m., madreporic plate ; t.p., plate which terminates Area V ; the 

 remaining symbols indicate the various areas according to Lov^n's system. 



3. Observations on the Minute Structure of the Spicules of 

 Calcareous Sponges. By E. A. Minchin, M.A., V.P.Z.S., 

 Professor of Protozoology, University of London, and 

 D. J. Reid, M.B., CM., F.Z.S. 



[Received May 12, 1908.] 



(Plates XXXIV.-XXXYII.*) 



Introduction. 



The minute structure of calcareous sponge-spicules has been 

 the subject both of much laborious investigation and of many 

 contradictory statements. In regard to the structure of siliceous 

 sponge-spicules investigators are practically agreed upon the 

 following points : — the mineral matter of the spicule, or spicule- 

 ray, as the case may be, forms a hollow tube with a relatively 

 thick wall ; in the lumen of the tube, termed the axial canal, is 

 lodged an axial filament of organic nature ; the siliceous tube 

 may be homogeneous in structure or may be sti^atified, that is, 

 composed of concentric layei'S of silica alternating with fine layers 

 of organic material ; and the outer surface of the siliceous tube is 

 enveloped in an outermost layer of organic substance forming a 

 sheath to the whole spicule. Thus in siliceous spicules we find, 

 apparently universally present, the following parts, passing from 

 within outwards : — (1) the axial filament, a definite structure 

 that can be isolated by hydrofluoric acid ; (2) the siliceous tube, 

 stratified or homogeneous ; (3) the spicule sheath. Turning now 

 to calcareous sponge-spicules, it is found tha,t the only point on 

 which all investigators are agreed is the presence of a sheath 

 enveloping the surface of the spicule. The following brief 

 historical summary of the statements that have been put forward 

 will make this clear. 



* For explanation of the Plates, see pp. 675-676. 



