STUDIES ON THE COMPARATIVE ANATOMY OP SPONGES. 163 



outer surfaces of the radial chambers and of the central tube 

 are doubtless caused by a layer of pavement epithelium, as in 

 other Heterocoela. 



Such, then, is the simplest type of canal system met with 

 amongst the Heterocoela; and, as it appears to me, all the 

 higher types met with in the group may be derived from 

 some such simple radiate one by modification in one or more 

 of the following ways : 



(1) By the outer surfaces of adjacent radial chambers 

 coming in contact with one another and fusing together. 

 This fusion at first takes place irregularly and partially, and 

 then more completely and throughout the length of the 

 chambers, so as to divide the water-containing space which 

 surrounds the chambers into a series of more or less well- 

 defined inhalant canals, sometimes called " intercanals " 

 (compare figs. 2 — 5). 



(2) By the closing in of the inhalant canals at their distal 

 ends by the outgrowth of a thin, pore-bearing, dermal mem- 

 brane from the walls of the radial chambers at or near their 

 distal extremities. In this way true " dermal pores " are 

 formed, through which the water gains access to the inhalant 

 canals (compare figs. 3, 6 — 8). 



(3) By the increase in thickness of the dermal membrane 

 and the development in it of a special skeleton, so as to form 

 a thick cortex, which not only stretches between but also 

 covers over the ends of the radial chambers. The formation of 

 such a thick cortex necessitates the development of a more or 

 less highly specialised '• cortical inhalant canal system," which 

 places the dermal pores in communication with the deeper 

 parts of the inhalant canal system (compare figs. 9 — 12, 23). 



(4) By the branching of the radial chambers, and conse- 

 quently of the inhalant canals which lie between them 

 (compare figs. 7, 9, 10, 18, 20). 



(5) By increase in thickness of the wall of the central 

 tube (gastral cortex), and consequently also in the length of 

 the exhalant canals (compare figs. 4, 9, 10). 



(6) By retreat of the collared cells towards the distal 



