XX 
THE VOYAGE OF H.M.S. CHALLEHGEE. 
increased development of mesoderm. At present we know next to nothing as to what cells 
in the sponge are concerned in the ingestion and digestion of food ; in an Ascon we may 
fairly suppose that it is the choanocytes of the endoderm, for no other endodermal cells 
exist ; in the higher sponges we have no reason to suppose that the choanocytes have 
lost this function, but at the same time we have no certain proof that the pinacocytal 
cells of the endoderm, which may be regarded as reduced or metamorphosed choanocytal 
cells, have lost it either ; they have ceased to be agents of propulsion, but not necessarily 
of ingestion and digestion, indeed it is possible that with the loss of the former function 
the latter has become enhanced. Next it has to be noticed that the mesoderm not only 
increases in relative volume, but undergoes a change of character, as the type of chamber- 
system is raised ; this change consists partly in its becoming more granular, the granules 
first appearing in the immediate neighbourhood of the flagellated chambers ; this granula- 
tion is indicative of more active metabolism, and is probably connected with the acquisition 
of nutriment ; it may be secretory, the mesodermal cells producing some solvent fluid, 
which subsequently enters the adjacent canals and breaks up the contained food into 
readily assimilated products, or it may be that the mesodermal cells are transformed 
epithelial cells, which, having obtained their share of food, have retired into the mesoderm 
to digest it. These and other conjectures, in the absence of exact knowledge, are open to 
us, but in any case we may probably connect the increased metabolism of the cells with 
alimentary processes ; and if so, the increased development of the mesoderm is to be 
connected with the more granular character it presents in sponges with aphodal 
chambers. To render this conjectural explanation clearer, we may add a hypothetical 
account of the succession of events which led from the eurypylous to the aphodal 
type of chamber-system, — a change which has occurred independently in different 
groups of sponges, and which must be susceptible of a physical explanation. Com- 
mencing with a eurypylous chamber, we may suppose that some of the choanocytes 
near its mouth became transformed into pinacocytes, which acquired increased powers 
of ingesting and digesting food, this led directly or indirectly to an increased growth 
of mesoderm in their immediate neighbourhood ; and consequently to a change in the 
form of the flagellated chamber, the apopyle of which became produced into a short 
tube or aphodus ; from this followed increased efficiency in current-producing power, 
eddies being reduced ; a further change in the same direction continued till the 
relative dimensions of chamber and aphodus were those of maximum efficiency. 
Diplodal Type op Chambee-System. 
The final stage in the modification of the chamber-system is that in which the in- 
current canals, as well as the excurrent, are encroached upon by the mesoderm. The 
mesoderm increasing in thickness, reduces the lumen of the incurrent canal and the 
