FORMS OF SPICULES. 71 



spicules^ or, rather, on the scleroblasts which deposit them, 

 and this seems still more probable when we remember that 

 scleroblasts are constituted of a soft and highly complex 

 sensitive substance which must be readily influenced by 

 mechanical and other forces, which are in all cases transmitted 

 through the gelatinous matrix surrounding the spicules. 



Further, in considering mechanical factors as applied to 

 biological phenomena, ''mechanists" are too apt to forget 

 that the substance of organisms is after all living, in other 

 words, possesses among other features the capacity of " spon- 

 taneously " altering its configuration within certain limits. 

 For example it is quite possible that the spinous processes 

 and elongated and branching forms of many spicules are 

 attributable to the pseudopodial activity of the scleroplasm 

 resulting from physiological requirements, and it is certain, 

 as I have elsewhere stated, that the perforate character of 

 most echinoderm plate-spicules and of radiolarian shells is 

 due to the necessity for communication across the area occu- 

 pied by the spicule or skeleton, and is probably determined 

 ontogenetically in each case, though exactly how it is diflfi- 

 cult to say. But at present I have not the space to do more 

 than suggest this form of activity of the protoplasm as a 

 possibly important factor in the production of spicule forms. 



I may finally remark that even the crystal-like symmetry 

 of some (certainly not of most) spicules (in Calcarea and 

 aplacophore Mollusca) can be referred to purely mechanical 

 conditions,^ as I have pointed out in Studies land VI. How- 

 ever, although I believe that many individual features of 

 spicules can be attributed to purely mechanical causes, yet it 

 is quite evident that factor (a) in all its aspects is but a sub- 



^ Tlie fact stated by Sollas [25] with reference to the spicules of calcareous 

 sponges (aggregates of calcite crystals), viz. that " the position of the rhom- 

 bohedra relative to the surface of the spicules is very similar to that which 

 may be observed in rhombohedra of calcite filling up a cavity within a lime- 

 stone rock, or inside the chamber of an ammonite," is suggestive. Sollas 

 adds that "we must suppose that the deposition of calcite withiu the spicule- 

 sheath occurs according to just the same laws which are followed in the 

 purely mineral world." 



