MATERIALS FOR A MONOGRAPH OF THE ASCONS. 547 



angles is to be explained in much the same manner as in the 

 well-known instance of the honeycomb. The triradiate systems 

 are, in fact, the form of spicule best fitted to support the porous 

 wall of the Ascons, and the anatomical structure and arrange- 

 ment of the soft parts show the utility of the specific structure 

 of the skeletal elements. A similar theory is worked out by 

 Schulze with great ingenuity and thoroughness for the primi- 

 tive tetraxon spicule of the Tetraxonia and the triaxon spicule 

 of Hexactinellids. In the case of the latter his view certainly 

 receives great support from the structure of the Cambrian 

 Protospongia, the oldest known fossil sponge. 



Sollas, in his monograph of the Tetractinellida (1888), made 

 an attempt to explain the forms of spicules as the mechanical 

 result of forces acting upon them during their growtb. After 

 claiming that the forms of bones or other skeletons in higher 

 animals are due to the effects of pressure and tension exerted 

 upon them during their growth by the surrounding tissues, he 

 thinks (p. Ixxv) that " were it possible to connect the special 

 forms of spicule with these forces an explanation would be 

 reached which would fulfil^' the necessary conditions; namely, 

 that a theory to explain the forms of spicules should be "in- 

 dependent of the nature of the material, and capable of being 

 applied to all organisms in which spicular forms are developed." 

 The principle of this explanation is "that all spicular struc- 

 tures tend to grow along lines of least resistance." " The 

 simplest form of spicule is a minute granule, generally more 

 or less spherical," and from this as a starting-point Sollas 

 derives the various forms by the aid of his theory. We need 

 not follow here in detail the ingenious manner in which the 

 principle of "least resistance" is applied to explain the various 

 spicular forms of Tetractinellids and other sponges. For the 

 calcareous sponges he says (p. Ixxxi), "Let a scleroblast be 

 situated near the surface of a sponge, as it must be in the 

 Ascones ; the surface tension will here also lead to the growth 

 of three actines inclined at angles of 120° to each other, and 

 thus the triradiate spicule so common in the calcareous sponges 

 may have arisen." 



