FORMS OF SPICULES. 75 



alveolar structure of calcareous and siliceous spicules described 

 by Biitsclili (3) has anything to do with the globulites of 

 crystal formation. 



Whatever may be the value of the above suggestions^ one 

 fact is certain^ viz. that prolonged investigation (largely ex- 

 perimental) on the entire subject is absolutely essential before 

 we can hope to interpret in at all an exact manner the shapes 

 of spicules. 



Note. 



One important and, indeed, vital source of evidence 

 respecting the inheritance or otherwise of spicule forms, 

 which I have left almost unconsidered in the text, lies in the 

 answer we must give to the question as to whether modifica- 

 tions of spicule forms, evidently due to ontogenetic causes, 

 ever appear in the development of the spicule apart from 

 these causes. Obviously, if this can be shown to be a fact in 

 a single instance, then the inheritance of this particular 

 spicule form is proved, and that of others rendered probable, 

 though how the arguments I have advanced above in opposi- 

 tion to this are to be met I fail to see. Only one such 

 apparent example of the inheritance of spicule forms, which 

 might be cited in evidence against my position occurs to me 

 just before returning the corrected proofs of this essay, and 

 this example is taken from a paper I have myself published. 

 In the first of my Studies in Spicule Formation (Sycon 

 sponges) I have suggested that the elongation of the "pos- 

 terior " ray in certain Sycon triradiates is directly due to its 

 vertical basal position, the stresses incident upon the sponge 

 being largely borne by the longitudinal element of the 

 skeleton, especially at the sponge base, and the actinoblasts 

 of the basal ray being thus stimulated to secrete more actively. 

 Now it so happens that in Sycon ciliata and some other 

 sponges the superior size of one of the rays is apparent in the 



observations of his own on tlie formation of clioHne crystals, in which 

 similar phenomena are described. 



