STUDIES IN SPICULE FORMATION. 49 



The spicules of Proneomenia, from their first appearance 

 each as a small needle contained within a single cell (figs. 7, 

 8), grow in a more or less vertical direction, and soon burst 

 through the cell-membrane (fig. 9). From this stage onward 

 the further growth of the spicule is confined to the basal 

 extremity (so resembling the growth of a hair), which alone 

 is enveloped by the cell-substance (fig. 10). An axial cavity 

 appears in the spicule substance before the spicule is half 

 grown, but closes up proximally before the full size is 

 attained. Many of the scleroblasts attached to the larger 

 spicules become more or less withdrawn from the hypodermis 

 (text-figare, b, c), and subsequently also lose connection with 

 the spicules themselves, which, perhaps owing to contrac- 

 tions of the integument, often come to lie, Avlien full grown, 

 near the outer limit of the cuticle (k in text-figure). 



In Dondersia the monaxon spicule, instead of being ver- 

 tical in position as in Proneomenia, is disposed more or less 

 horizontally from the first (figs. 11, 12) in correspondence 

 with the thin cuticle (thin as compared Avith that of Pro- 

 neomenia, being little more than the thickness of the hypo- 

 dermis). These spicules also when adult become sepai-ated 

 from the hypodermis, and lose their scleroblasts. 



I may add that the figures and statements of Heuscher 

 and Hubrecht, affirming the multicellular papillary origin of 

 the spicules of Proneomenia, are misleading, though super- 

 ficially they appear to be correct. They have solely resulted 

 from insufficient attention being paid to detail, and, bearing 

 in mind the facts described above, it will easily be seen how 

 the mistake has arisen. 



It may also be mentioned that the hypodermis contains 

 many gland-cells with mucilaginous contents, which have 

 been, on at least one occasion, figured as young spicules ! 

 Needless to say the two are readily distinguishable. 



With reference to the mode of growth of these spicules of 

 the Aplacophora, I may here point out that the production 

 of a straight '*■ finished^' mineral structure by terminal 

 accretion due solely to the activity of a single scleroblats 



VOL. 51, PART 1. NEW SEEIES. 4 



