492 ^Iv. A. T. Mastevmaii on the Nutritive and 



A similar intracellular excretion is well known to exist in 

 many Metazoa, as shown, for instance, by Durham in Asterias 

 and by other observers in FhyUirhoe, Bipennaria^ &c. 



The clear recognition of this intracellular excretion as 

 occurring throughout the Metazoa concurrently with the 

 liquid intercellular excretion would perhaps go far to explain 

 the mor])hological differentiation of the excretory organs in 

 this group, and seems to have a direct bearing upon the origin 

 and significance of the coelom. 



The " ncphrocy tes " find their way to the exterior, so far as 

 can be seen, mainly through the " ectoderm " or outer limiting 

 layer of the colony ; but there is no doubt that many also 

 break through into the paragastric cavity, or even into the 

 incurrent canals. A number of these loaded nephrocytes may 

 be seen lying just under the outer layer, and they might 

 under some circumstances, as referred to above, be mistaken 

 for amoeboid cells which are performing the function of 

 ingesting carmine. 



From the above facts we can now summarize the processes 

 of digestion and excretion in Grantia compressa as follows : — 



(1) Ingestion of food-particles into the substance of the 



choanocytes, or, at any rate, mainly into them, the 

 possibility of their absorption by other parts, such as 

 the ectoderm or endoderm, being undoubted, but prac- 

 tically of no importance compared with the former. 



(2) ]\lor|)hological transformation of the choanocytes into 

 amocbiform cells, in no way differing in appearance 

 from so-called mesoderm-cells. 



(3) Immigration of these cells to interior, where intracellular 

 digestion takes place. This stage is probably con- 

 current with stage (2). 



(4) Replacing of the immigrated cells by fresh choano- 



cytes arising from mor])hologically transformed meso- 

 derm-cells. 



(5) Excretion of the solid waste particles by amoeboid 

 nephrocytes, which burst tlirough the limiting layer of 

 pinnacocytes (mostly the outer layer, but not confined 

 to that part) and leave the colony, probably to disin- 

 tciirate. 



In very briefly comparing these results with those of other 

 observers wc note that Carter (2) describes having traced the 

 course of carmine-particles in the young Spongilla, and 

 remarks that they are absorbed by the '' spongozoa " (choano- 

 cytes), and that at the expiratioi\ of fifteen minutes the 

 particles are again set iVee by these cells. Although he does 

 not speak with any great certainty upon the course taken by 



