REPORT ON THE TETRACTINELLIDA, 
XIX 
a remnant of the cavity of an excurrent sinus which has been left behind amidst a 
surrounding growth of mesoderm. The accompanying diagrams (Fig. V.) will serve to 
make this clear. 
The aphodal character appears at a very early stage in the embryological history of 
those sponges in which it occurs ; it is already perfectly displayed, for instance, in a young 
sponge of the species, Stelletta pJirissens, not measuring more than 0'65 mm. in diameter 
(PI. XVI. fig. 18); at the same time some of the chambers in this example retain a 
eur 3 ^ylous character. 
The change in the character of the mesoderm already alluded to as accompanying the 
transition from the eurypylous to the aphodal chamber-system, consists in its conversion 
from collenchyma to sarcenchyma (see p. xxxviii). In young sponges up to at least 3 mm. 
in diameter the whole of the choanosomal mesoderm consists of sarcenchyma ; but in 
the adult sponge the mesoderm forming the walls of the larger excurrent canals is 
collenchymatous. 
In some of the lower forms of sponges with aphodal chambers ( simplex, p. 17), 
where the aphodus is very short, the collenchyma merely becomes abundantly granular in 
the neighbourhood of the flagellated chambers, but in the higher forms with a fully 
expressed aphodal chamber-system, and these are the rule in the Tetractinellida, it is 
entirely replaced by a true sarcenchyma. 
One other important modification remains to be mentioned ; this is the relatively 
smaller size of the flagellated chambers that usually marks the aphodal as compared with 
the eurypylous chamber-system. A similar reduction accompanies the passage from the 
simpler to the more complicated types of eurypylous chamber-system. From these 
generalisations we reach one still more general, namely, that the flagellated chambers 
diminish in size correspondingly with increased development of the mesoderm. 
In the absence of any exact knowledge of the physiology of sponges it would appear 
hopeless to attempt to explain this result, but the following considerations present them- 
selves ; — In the eurypylous type of chamber-system, the choanocy tes work at a mechanical 
disadvantage, no inconsiderable part of their energy being expended in producing eddies, 
which in no way promote but rather interfere with the production of currents deflnitely 
directed ; in the aphodal type this useless expenditure of energy is to a great extent 
prevented by the elongation of each chamber into a tube, which isolates the separate 
currents and prevents them from interfering with each other. Increased efficiency being 
thus acquired, the same amount of current can be produced by fewer choanocytes ; and 
thus smaller chambers, and relatively to the bulk of the sponge fewer choanocytes, 
characterise the aphodal as compared with the eurypylous chamber-system. Until the 
volume, velocity, and duration of the currents in different types of sponges has been 
investigated, this explanation must be regarded as hypothetical ; and still more so the 
following attempt to connect the change in the character of the chambers with the 
