556 E. A. MINOHIN. 



For all these reasons I am most decidedly of opinion that 

 Dreyer^s theory of a primitive tetraxon spicule, produced by 

 the influence of vesicular tension upon a sclerite deposited at a 

 nodal point of the protoplasmic framework, must be rejected 

 for calcareous sponges at least. My own observations upon 

 the formation of the spicules seem to me to indicate clearly 

 that the primitive form of skeletal element in the 

 Calcarea was a rod-shaped or fusiform sclerite ; that 

 the triradiate system has arisen from a junction 

 of three such simple elements ; and that the quadri- 

 radiate system, the last stage in the evolution, and 

 not the first, was further built up by the addition of 

 yet another monaxon sclerite to the triradiate 

 system. The primitive monaxon sclerite probably differed in 

 one point from the ordinary monaxon spicules that we are familiar 

 with in existing Calcarea. The latter always project free from 

 the surface of the sponge, only the proximal portion of the 

 spicule being embedded in the sponge wall. In the triradiate 

 systems, on the other hand, the rays are placed longitudinally, 

 and completely embedded in the body-wall, at least in all the 

 more primitive Ascons. Hence it is probable that the ances- 

 tral monaxon spicules lay tangentially in the body-wall, and 

 did not project from it. The spicules of this character under- 

 went two divergent courses of evolution. Some remained 

 single, but acquired a portion projecting free from the surface 

 of the dermal layer, becoming the existing monaxon spicules. 

 Others again acquired no such projecting portion, but, remain- 

 ing completely embedded in the body-wall, became united with 

 one another in trios to form the triradiate spicules as we know 



author notices their relation to the protoplasmic structure. The formation of 

 sclerites in plants — the so-called raphides— offers the best instance to the 

 point. Vacuoles of mucilage arise in the cell protoplasm, and run together to 

 form a large vacuole in which the raphides are deposited as crystals. See 

 Haberlandt, ' Physiologische Pflanzenanatomie (Leipzig, 1896), p. 449, figs. 

 190, 191; de Bary, 'Comparative Anatomy of the Phanerogams and Perns' 

 (Oxford, 1884), pp. 137—139; Gardner and Ito, "On the Structure of the 

 Mucilage-secreting Cells, &c.," ' Annals of Botany,' i (1887), pp. 27—54, 

 pis. iii, iv. 



