STUDIES IN SPICULE FORMATION. 47 



careous matter derived from the scleroblast cytoplasm which 

 entirely surrounds the spicule. The median portion of the 

 adult spicule is formed fii-st (evident when this is thickened), 

 the tapering extremities growing out from this. The sclero- 

 blast, i. e. single nucleus, never divides, so that the spicules 

 are purely unicellular products, and in most cases the body 

 of the scleroblast (the small mass of protoplasm immediately 

 surrounding the nucleus) is constantly situated midway in the 

 length of the spicule, i. e. in the vicinity of the thickening in 

 those spicules possessing this feature. 



Spicules in Aplacophorous Moll us ca. 



My material consisted of specimens of Proneomenia 

 aglaophenite and Dondersia banyulensis, specially 

 prepared at Plymouth by the osmic acid and picro-carmine 

 method which gave excellent results. 



So much has been previously written and so many good 

 figures provided in connection with the development of these 

 characteristic calcareous spicules of the Aplacophora that 

 my sole excuse for re-considering the subject is the uncertainty 

 which still prevails as to whether tliese spicules are unicelki- 

 lar (Thiele [8], Wiren [9]) or multicellular (Heuscher [3], 

 Hubrecht [4], Kowalevsky and Marion [5] , Pruvot [7]) in 

 growth. There is also a misapprehension to correct, which 

 is that the spicules embedded in the cuticle of Proneomenia 

 are ''in relation internally with epithelial papilias" (Sedgwick's 

 ' Text-book of Zoology,' p. 353, likewise Pelseneer's 'Mollusca' 

 in Lankester's ' Treatise on Zoology '). 



To state the results of my direct observations and inquiries 

 ns briefly as possible, I may say that it is now quite certain 

 that the spicules of the Aplacophora, like those of the Poly- 

 placophora (Pelseneer), all arise individually iu a single cell 

 of the hypodermis (figs. 7 — 13). It is also certain that the 

 spicules in Proneomenia (and other Aplacophora which pos- 

 sess an integument of similar type) do not in the majority of 

 cases bear any relation to the hypodermal papillas, and that 



