250 Mr. W. J. Sollas on the 



shown in figure 6 gives one the idea that the canals marked e' 

 are distributive and not collective in function, in which case 

 the water, which had already passed from the incurrent into 

 the excurrent canals, would be again distributed through a 

 fresh set of ciliated chambers, and thus be used twice over. 

 This does not appear probable. The tubes el have all the 

 characters of excurrent tubes — widely perforated trabecular 

 walls with dichotomous canals opening into them ; while the 

 tubes i' are equally incurrent in character and give off 

 canaliculi, which enter the ciliated chambers in the abrupt 

 fashion so characteristic of incurrent canaliculi. To complete 

 our representation of the canal-system we must therefore sup- 

 pose that the incurrent canals i' are connected in a round- 

 about way by concealed canals with the subcortical crypts. 

 By this supposition a double using of the incurrent water is 

 avoided. In the centre of the sponge one observes sections of 

 canals cut across in every possible direction ; but even here 

 the distinctive characters of the excurrent and incurrent canals, 

 as described above, appear to be maintained. 



The general course of the water-circulation of the sponge 

 would appear to be as follows : — The water finds access 

 through the dermal pores or ostia to the chones, whence 

 it finds it way into the subcortical crypts and the incurrent 

 canals ; from these it is distributed by multitudinous little 

 canals to the ciliated chambers, the seat of the energy on which 

 the working of the water-circulation depends. From these 

 chambers it passes out by fine canaliculi, which, after uniting 

 together once or twice or oftener, empty themselves into the 

 trabecular excurrent tubes ; from these the water flows unob- 

 structed into the large excurrent vessels, which deliver it 

 through sphinctral apertures into the oscular tube, whence it 

 passes freely to the exterior. 



Histology. 



1. The Cortex. — The exterior of the sponge is covered by a 

 thin membranous film, immediately beneath which is a single 

 layer of minute cells (PL XII. fig. 26), each containing a 

 minute cylindro-stellate spicule, and having an average dia- 

 meter of from 0*0002 to 0'0003 inch. The superficial mem- 

 brane appears to be a mere secretion of the underlying cells, 

 and with them forms the epidermis of the sponge (PI. XII. 

 fig. 26, e). 



The epidermis is succeeded by a layer of curious tissue 

 (PI. XII. fig. 26, c), which presents a striking but superficial 

 resemblance to the parenchymatous tissue of plants. It con- 

 sists of an irregular network of very refringent, faintly bluish, 



