56 W. WOODLAND. 



growth,^ we may define a spicule as a liard, crystalline or 

 colloidal deposit, of more or less extended and 

 often definite and complex form, always possessing 

 curved surfaces and never plane facets, formed 

 initially witliin a cell or a cell-fusion, and whose 

 subsequent growtli, which may be intra- or extra- 

 cellular, is due either solely to the activity of the 

 mother-cell or cells and its or their division-pro- 

 ducts if formed, or also partly to the activity of 

 cells not derived from the original mother-cell or 

 cells. Spicules which originate as a single deposit in the 

 interior either of a single cell or of two or more cells which 

 are more or less fused (and by a " cell " we mean a nucleus 

 associated with a mass of protoplasm whose limits may or 

 may not be distinguishable from other similar cells, thus con- 

 sidering a syncytium as a collection of cells) and whose sub- 

 sequent growth is solely associated with this cell or these 

 cells or its or their division-products are termed simple 

 spicules (spicules of Spongilla and apparently most other 

 Monaxonida, many tetraxonid spicules, radiolarian spicules, 

 alcyonarian spicules, monaxons of calcareous sponges, many 

 holothurian spicules, etc.) ; simple spicules which arise in 

 juxtaposition, and which subsequently become fused so as to 

 form a complex whole of a higher order of individuality (e.g. 

 triradiates of calcareous sponges, the rosettes of the Esperia 

 larva and possibly the plate-and-anchor spicules of Syuaptida;) 

 give rise to what may be termed aggregate spicules; finally, 

 spicules whose later growth is in part effected by adventitious 



' By iutra-cellular growtli we mean tiiat tlie deposit is enclosed on all sides 

 by the substance of the cell, as is the case in most spicules. In Calcarca, how- 

 ever, the apical actinoblasts, e. g. of the calcareous triradiates, are cylindrical 

 structures enclosing the rays at their a])ical extremity and a certain portion 

 of their length, and it is therefore clear that the deposits which lengthen the 

 rays are not intra-cellularly produced as above defined, since they are not 

 entirely surrounded by the cell; and from this cylindrical disposition of the 

 scleroblast to the simple adhesion of the cell to the surface of the spicule 

 there is obviously every transition. 



