EOKMS OF SPICULES. 55 



A Preliminary Consideration as to the possible 



Factors concerned in the Production of 



the various Forms of Spicules. 



By 



\¥. ^tVoodlaiid, 



Demonstrator of Zoology, King's College, London. 



So many misconceptions prevailing as to the essential 

 nature of spicules, and in consequence as to the causes which 

 can possibly give rise to the multitudinous and remarkable 

 forms which spicules assume, a brief consideration of the 

 subject seems to be called for. Such at least will serve to 

 remove sundry erroneous ideas which are continually being 

 put forward in explanation of these phenomena and will also 

 point out the direction in which further investigation is 

 necessary for their better comprehension. 



Spicules Defined, 



Recalling to mind among the more obvious features of 

 spicules that these bodies (a) always possess a form other than 

 the spherical (otherwise being termed concretions), {h) always 

 possess curved surfaces and never plane facets (so being dis- 

 tinguished from crystals whether these be simple or aggre- 

 gate), and (c) that, although always intra-cellular in origin 

 and usually so during their whole subsequent development, 

 yet certain spicules (calcareous spicules of sponges, plutei and 

 aplacophore Mollusca, and some siliceous sponge spicules) 

 are extra-cellularly produced during the later part of their 



