542 E. A. MINCHIN. 



the spicules^ a mode of formatioa analogous to that supposed 

 to occur in siliceous sponges, has been universally assumed. 

 Two notions have dominated all the writings on this subject : 



(1) that spicules must arise each in a special mother-cell; 



(2) that their subsequent growth must be aided by other cells 

 of the body-wall. To these principles some authors add a 

 third, as obviously based on the analogy of siliceous sponges, 

 namely, that the tetraxon spicule must be the primary form, 

 from which the others arise by reduction of the number of 

 rays. I shall discuss the last of these three propositions more 

 fully in the theoretical portion of this paper, and hope to show 

 with regard to it what is sufficiently obvious in the case of the 

 first two, namely, that a priori reasoning, founded on pure 

 analogy, has led to conclusions hopelessly at variance with the 

 facts. 



{b) Observations upon the Structure and Compo- 

 sition of the Calcareous Spicules. — The chemical and 

 physical nature of the calcareous spicules has been so tho- 

 roughly worked out by Haeckel (1872), SoUas (1885), and 

 Ebner (1887), that as I have nothing to add to the sub- 

 ject, I shall content myself with the references to these 

 authors, and a brief summary of the results reached by the 

 careful investigations of Ebner as a preliminary to a theoretical 

 discussion. 



The most striking, and in many ways unexpected peculiarity 

 of these bodies, of which the triradiate and quadriradiate forms 

 are so clearly shown by their development to be compound 

 structures, built up from different sources, is the fact that 

 each spicule behaves optically as a single crystal. This was 

 first shown by Sollas and confirmed by Ebner, who further 

 denied the presence of organic matter in them, postulated by 

 Haeckel under the name of spiculin. " Each spicule behaves 

 as a single crystal individual, and an organic substance cannot 

 be demonstrated in it. . . . The spicule by no means consists 

 of pure calcium carbonate in the form of calcite, though very 

 similar to it from the point of view of crystallography (in 

 krystallographischer Beziehung), but the spicule substance 



