34 THE president's address. 



into contact with one another more or less throughout their 

 length and fusion takes place. Thus the originally continuous 

 system of interspaces between these chambers becomes broken 

 up into more or less sharply defined intercanals, or inhalant 

 canals, through which water gains access to the prosopyles 

 in the chamber walls. The openings of these inhalant canals 

 appear as gaps between the blind distal ends of the radial 

 chambers. 



The next important stage in the evolution of these sponges 

 is the development of a dermal cortex, and it is not difficult 

 to see how this has taken place. We find the beginning of the 

 process in some species of Sycon, in which the external opening 

 of each inhalant canal becomes covered over by a delicate 

 membrane pierced by secondary inhalant pores, or dermal 

 pores, analogous to the prosopyles in the walls of the radial 

 chambers but quite different in their mode of origin. This dermal 

 membrane is an entirely new development, but it has evidently 

 been formed in a very simple manner by extension of the outer 

 layer of the walls of the radial tubes. It is at first very thin 

 and delicate, without any supporting skeleton of its own, and 

 merely serves as a kind of sieve for filtering the stream of water 

 as it enters the sponge and perhaps, by appropriate contraction 

 of the dermal pores, as a means of regulating the rate of flow. 

 Its appearance, however, gives rise to new possibilities in the 

 way of skeletal development. These are realised chiefly in the 

 appearance of a definite cortical skeleton, composed principally 

 of triradiate spicules arranged tangentially as regards the dermal 

 surface. The development of such a dermal cortex is charac- 

 teristic of all the higher families of Calcarea. 



With the appearance of both gastral and dermal cortex new 

 mechanical principles come into play in the formation of the 

 skeleton. Up to this point the skeleton of each radial chamber, 

 or tubar skeleton, as it is termed, has been an independent 

 structure developed in the walls of that chamber and in relation 

 to that chamber alone. Now, however, that the originally 

 separate chambers have all become intimately connected together 

 so as to form, with the dermal and gastral cortex, a kind of 

 secondary, and of course much thicker, wall to the central gastral 



