68 the president's address. 



fact, especially when we consider how very widely it is afterwards 

 departed from on most lines of spicule evolution. It has been 

 suggested that the equiradiate and equiangular tetraxon was 

 originally adapted to the interstices in a system of spherical 

 flagellated chambers arranged tetrahedrally. This seems probable 

 enough, but in any case we can safely take this form as our 

 archetype without indulging in speculations as to its origin. 



If we now imagine one ray of our archetype becoming greatly 

 elongated we get a common form of " triaene " spicule known 

 as the " plagiotriaene " (fig. 2), with a long arm or " shaft " 

 and three short arms or "cladi," but still with all the angles 

 equal. If we imagine the angles which the cladi make with 

 the shaft to be increased, so that the cladi come to point forwards, 

 we get the "protriaene" (figs. 5, 5a); if the cladi extend at 

 right angles to the shaft we get the " orthotriaene " (fig. 3), and 

 if they point backwards we have the " anatriaene " or grapnel 

 spicule (figs. 4, 4a). 



All these long-shafted triaenes are typically oriented with 

 the cladi at or near the surface of the sponge, and the shaft 

 directed centripetally inwards, so that the entire skeleton acquires 

 a markedly radiate arrangement. The cladi of the orthotriaenes 

 usually form a support for the dermal membrane at the surface 

 of the sponge, beneath which they are spread out tangentially, 

 and their efficiency as a dermal skeleton may be greatly increased 

 by their bifurcation (" dichotriaenes," figs. 6, Ga). In the case of 

 the protriaenes and anatriaenes the distal portions of the shafts, 

 bearing the sharp-pointed prongs or cladi, usually project for 

 some distance beyond the surface of the sponge, and in this 

 position they probably serve either to ward off the attacks of 

 enemies or to entangle minute organisms whose decomposition 

 may supply the minute organic particles upon which the sponge 

 depends for its food supply and which will be carried inwards 

 by the inflowing stream of water. 



A still more remarkable modification is met with in the 

 " discotriaene," in which the shaft is reduced to a short peg 

 inserted in the middle of a flat disk formed by fusion of the 

 cladi. The entire spicule then assumes somewhat the form of 

 a carpet-nail. In the genus Discodermia we find these disco- 

 triaenes stuck close together all over the surface of the sponge,, 

 and forming an impenetrable mail-armour. 



