650 E. A. MINOHIN. 



that the triradiate spicule is not a simple and primitive 

 skeletal element, but is built up from three monaxon spicules ; 

 in fact, that it arises in just the manner which Haeckel dis- 

 missed without further discussion as improbable and un- 

 founded. It is, therefore, clearly impossible that the triradiate 

 system should owe its form, as a whole, to the action of 

 crystallisation. 



While unable to accept biocrystallisation as an explanation 

 of the primitive forms of the calcareous sponge spicule, I fully 

 agree with the views Haeckel has put forward with regard to 

 their evolution. With Haeckel I consider that there are two 

 primitive types of spicule in the Calcarea, the monaxon and 

 the triradiate, which are independent of one another in the 

 form in which we now meet with them, though I shall try to 

 refer both to modifications of a yet simpler monaxon type. 

 With Haeckel I believe that these two forms of spicule are 

 associated with two lines of descent in the Calcarea, the one 

 starting from forms such as those constituting Haeckel's genus 

 Ascetta, the other from his genus Ascyssa; and I may refer 

 to my own later attempts (1896) to give this theory further 

 support, and to introduce a classification of the Homocoela 

 modelled upon it. With Haeckel, finally, I am fully of the 

 opinion, of which I think the researches here brought forward 

 are a convincing proof, that the quadriradiate spicule is a 

 secondary form, derived, by the addition of an adventitious 

 ray, from the triradiate system. 



The view advocated by Schulze — namely, that the funda- 

 mental forms of the spicules are to be explained by adaptation 

 to the primitive types of structure in sponges — is the one which 

 seems to me to contain the true solution of the problem, at 

 least in the case of calcareous sponges ; and I shall presently 

 try to show that my investigations at once confirm and extend 

 Schulze's conclusions. The one defect, to my mind, in Schulze's 

 presentation of the case is that he left out of consideration 

 the monaxon spicules, which are certainly of as primitive a 

 type as the triradiates. No scheme of evolution seems to me 

 complete which fails to indicate the relations of the monaxon 



