THE president's ADDRESS. 37 



has once been adapted in relation to the sponge-wall as a whole 

 rather than to the individual radial chambers, the canal-system 

 may undergo further evolution without affecting the skeleton 

 arrangement. This is true not only of the Heteropiidae and 

 Amphoriscidae, but also of the Grantiidae, In all these families, 

 by a complex process of folding of the chamber-layer as a whole, 

 sometimes accompanied by branching of the chambers them- 

 selves and reduction in their size, the radial arrangement may 

 become lost and the well-known transition from the syconoid 

 to the sylleibid and perhaps even the leuconoid types of canal- 

 system be effected. 



The entire series of skeletal modifications which we have so 

 far considered are evidently of an adaptive character, suited to 

 meet the new requirements arising in the course of the evolution 

 of the sponge as a whole through the processes of colony- 

 formation and integration. Many other illustrations of the 

 same type of evolution might be selected from other groups of 

 sponges. In this connection I may again refer very briefly to 

 the evolution of tetraxonid siliceous spicules. In this group 

 we never find a canal-system with elongated flagellate chambers 

 arranged radially around a central gastral cavity, and the 

 siliceous skeleton does not seem to have made its appearance at 

 all until the canal-system had reached a stage corresponding to 

 the most advanced type of arrangement met with amongst the 

 Calcarea, with numerous small spherical chambers scattered 

 through the thickness of the sponge-wall and a branching system 

 of inhalant and exhalant lacunae. It has been supposed that 

 the primitive tetraxon spicules were originally developed in 

 the three-dimensional intervals between the chambers, just as 

 I have suggested that the primitive triradiates of the Calcarea 

 were developed in relation to the two-dimensional intervals 

 between the prosopyles of a primitive Leucosolenia. However 

 that may be, the primitive skeleton of scattered tetracts appears 

 to have given rise along various lines of evolution to an immense 

 variety of skeletal types both as regards spicule form and spicule 

 arrangement. Many of these types are as clearly adaptive in 

 character as in the Calcarea. I need only mention such cases 

 as the development of triaenes and their specialisation for 



