218 



emigration has been worked out mainly by following up the fate of 

 various pigments, after their introduction into the organism. 



In the same way, as in the special alimentary processes, the mono- 

 cytic skeletons, and other organs, may be replaced by poly cy tic, such 

 as hairs, etc., which arise by the activity of several tissue cells, and 

 their rudiments by polycytic invaginations ^^. 



Excre tio n. 

 In the consideration of true excretion of waste products of meta- 

 bolism, it is obvious that the whole protoplasm of an organism must 

 eliminate liquid waste products as an essential of its existence, but 

 granted a ready interchange from cell to cell, then the cells in contact 

 with the exterior will be those in which excretory organs make their 

 appearance. The outer lining membrane or ectoderm has the advan- 

 tage of position over the endoderm in quickly getting rid of the waste 

 products so that in the latter will be developed only those excretions 

 which can be utilised by the organism either directly or indirectly in 

 the processes of digestion, and confined to the former (ectoderm) will 

 be the function of getting rid of waste products which cannot be uti- 

 lised in any way by the organism. Thus the lowest form of excretory 

 organ will consist of the whole outside limiting ectoderm as in the 

 case of sponges, the secretory activity of the ectoderm cells contin- 

 ually removing, from the body-fluid, the liquid waste products of the 

 whole colony. 



From what has already been said regarding the monocytic digestion 

 in monoblastic forms, it is evident that in such form as in Fig. 1 

 digestion being efi'ected by the cells which have migrated to the centre 

 the nutritive fluid set free by this process will difl'use from the centre 

 outwards and as one reaches the outer layer the degradation products 

 (or katabolic residua) will be predominant and will be removed by the 

 secretory activity of the outer layer of cells or by simple diifusion 

 through them to the exterior. 



Somewhat the same distribution will occur in the higher diplo- 

 blastic type (Fig. 2) the main difi'erence being that here the beginning 

 of polycytic digestion will set free digestive fluids in the enteric cavity 

 which, difi'using through the endoderm, will supplement the digestive 

 fluids arising from the monocytic digestion. (The nutritive fluid 

 arising from the polycytic digestive activity is indicated Fig. 2 and 

 Fig. 3.) 



In the triploblastic type (Fig. 3), as already indicated, the mono- 

 cytic ingestive cells migrate inwards from the endoderm into the coe- 

 sa See Note I. above. 



