207 



phylogeny a complete invagination is effected, an accelerated imitation 

 of the same occuring in ontogeny: precisely the same migratory in- 

 gestive movement accounting for both the evolution of endoderm 

 and of the mesoderm. 



In poly cy tic ingestion, a further area of tissue cells becomes sub- 

 servient to ingestion, and forms a special pseudo-endodermal area, 

 known to morphologists as the stomodaeum. 



The significance of the distinction between monocytic (intracel- 

 lular) and polycytic (extracellular) digestion is well known, but the 

 part which must be emphasised here is that in the evolution of the 

 digestive processes of the Metazoa the unattained ideal is the perfec- 

 tion of the polycytic digestion to the entire exclusion of monocytic 

 processes ^^. Although in the digestion of proteids and carbohydrates 

 the polycytic method becomes predominant yet the monocytic ingestive 

 and digestive processes survive in the highest types in connection 

 with the absorption of fats 20 and in the phenomena of phagocytosis 2^. 



The perfection of the polycytic digestion is then the key-note of 

 the principles upon which the metazoan digestive system is elaborated. 

 On the other hand, at the base of the metazoan tree is the sponge 

 group in which there is no polycytic digestion whatever 2-2. The pores 

 are polycytic digestive organs, and the canals represent a polycytic 

 enteric cavity; to some extent the osculum may be regarded as 

 a polycytic anus. All food particles, however, which are made use 

 of by the sponge must be absorbed by each flagellated cell , sepa- 

 rately, and thus monocytically. In fact the Porifera are to be regard- 

 ed in this respect as colonies of Protozoa that the ingestive and 

 digestive functions are carried on, monocytically, by the independent 

 and individual energies and actions of single cells lining the inner 

 cavities. Interdependence between the respective cells of the colony 

 only commences when digestion has been effected and a distribution 

 of the liquid products of digestion takes place. This distribution of 

 assimilated material to all the colony allows of a certain amount of 

 specialisation in certain cells to sexual, skeletal and protective func- 

 tions, which feature brings the Porifera into line with the protozoa. 



19 For review of the work on monocytic and polycytic digestion in Coelen- 

 terata, see S. J. Hick son, loc. cit. p. 3. 



20 »Thus we arrive at the result that, in the lower Vertebrates, — and, with 

 certain limitations, in the higher types also , — active or mechanical processes take 

 place indigestion«. R. Wiedersheim, Comp.Anat. Vertebrates. — E. A. Schäfer, 

 Internat. Journ. of Anat. and Hist. Vol. H. P. I. 1885. 



21 M. A. Buffer, Q. J. M. S. XXXII and E. Metschnikoff, Zeitschr. f. 

 wiss. Zool. et Biol. Cent. etc. 



22 A. T. Masterman, loc. cit. p. 7. 



