546 E. A. MINCHIN. 



dently of the other two, since there are no transitions between 

 them so far as the skeleton is concerned^ and their common 

 ancestor can only have been a simple primitive form without a 

 skeleton. 



Schulze next discusses what is the most primitive form of 

 spicule in each of the three great groups into which sponges 

 are thus separated, and tries to show that the form of the 

 primitive spicule is in each case a direct adaptation to the 

 simplest type of anatomical structure, as seen in the more 

 primitive or least specialised examples of the groups, or in 

 young specimens. It would be out of place here to discuss 

 at length his explanation of the origin of the triaxon and 

 tetraxon siliceous skeleton, and we will only consider in detail 

 his theory as applied to Calcarea. 



Schulze does not consider any crystallisation theory as a 

 sufficient explanation of the form of a spicule as a whole : (1) 

 because symmetrical forms occur not only in the case of 

 spicules composed of a crystalline material such as calcite, but 

 also in the case of siliceous spicules composed of a colloid sub- 

 stance, namely, opal ; (2) because the rays frequently deviate 

 from the typical angles, and are often markedly curved. He 

 considers even the fundamental types of the spicules to be de- 

 termined solely by the matrix in which they lie. In Calcarea 

 the most primitive group is that of the Ascons, represented by 

 the olynthus stage in the development of the higher forms, 

 and the most primitive type of spicule is the regular triradiate, 

 with three equal rays meeting at equal angles of 120° in a 

 plane. An Ascon or an olynthus has the form of a thin- 

 walled tube, open at the free end, with the wall perforated by 

 uniformly distributed pores. The triradiate spicules occur 

 embedded in the wall, their rays lying tangentially to the 

 surface, each spicule being typically so orientated that one ray 

 is directed away fron^ the osculum, while the other two run 

 obliquely forwards, or rather upwards. In the angle between 

 any two rays a pore is situated,^ and the regularity of the 



1 Schulze suggests also an alternative arrangement, which to the best of 

 my belief, however, does not occur in nature, and need not be discussed. 



