Excretory Processes in Pari ft rn. 495 



become divided off to a greater dejjree into two morphological 

 types concurrently with a more complete division of physio- 

 logical labour — so much so that, as Metsclmikoff indicates 

 above, some forms occur in which there are choanocytic cells 

 performing otJi/ the function of loci^uiolion (of the j)articli's). 



These cells we should expect to find remaining constant 

 throughout lift', and they thus form a real e])it}h'lintn of cells. 

 The function of digestion, on the other hand, is here entirely 

 performed by the mesodermic cells. Ilonce within the sponge 

 group we find a process of physiological division of labour 

 connected with the processes of ingestion and digestion, which 

 may largely account for the morphological differentiation as 

 seen in the various types. The smaller the proportion of the 

 choanocytic endodermal area to the pinnacocvtic area the less 

 part (we shall expect to find) the individual cells of the former 

 will take in the digestive processes and the more they will be 

 differentiated into a definite "tissue" performing as a pre- 

 dominant function throughout life that of ingestion — that is to 

 say, interreJhilar ingestion into the canals of the sponge and 

 intercellular excretion through the osculum of the same, 

 whereas their former function was that of intracellular inges- 

 tion, digestion, and excretion. 



In the sponges, then, there is intercellular ingestion and 

 excretion, as in all Metazoa, but there is no intercellular 

 digestion, at any rate in the lower forms, a very important 

 feature in comparing tliem with the CcTelenterata. 



The Poritera would also appear to really consist of two 

 layers only, one definite and fixed — the ectoderm — and the 

 other, Metschnikotf's meso-endoderni, consisting of un- 

 specialized cells, any of which may give rise to ova, sperma- 

 tozoa, locomotive, excretory, skeletal, or digestive phases 

 which have their corresponding temporary morpliological 

 modifications. 



It is interesting to note that the probable process of diges- 

 tion indicated above shows a very low type of Metazoan 

 metabolic processes. Any one of many cells in close con- 

 tiguity engulfs solid food particles, which it reduces to the 

 liquid statCj or the parts of them that are capable of being so 

 reduced, by digestive processes. It then parts with most of 

 its liquid nutrient material to the surrounding cells by diffu- 

 sion or a modified process of a like nature, and is expelled or 

 emigrates from the colony, carrying with it the solid waste 

 products. 



This probably represents the scheme of all intracellular 

 digestion, and a modified form of it is found in the processes 

 of ingestion as conducted by the "yellow cells " of Annelida, 



