208 



These points may be summarised by a physiological classification of 

 animal organisms: 



1) Protozoa: Unicellular or multicellular with monocytic in- 

 gestion, digestion and egestion. 



2) Porozoa (Porifera): Multicellular animals with polycytic in- 

 gestion and egestion and monocytic digestion. 



(Many polycytic ingestive apertures.) 



3) Metazoa: Multicellular animals with polycytic ingestion, di- 

 gestion and egestion. 



(One polycytic ingestive aperture.) 



In the Porozoa and Metazoa all the monocytic processes also per- 

 sist to a greater or less degree. 



Just as polycytic ingestion is essentially connected with the 

 stomodaeum, so polycytic ingestion is connected with the endo- 

 dermal tissue, and in this tissue arise digestive glands. 



The fact that an animal absorbs solid food necessitates the elimi- 

 nation of the indigestible residue from the organism. This process is 

 known as Egestion. 



Turning to the Protozoa we find that monocytic egestion is effec- 

 ted by simple contraction of the protoplasm resulting on the extrusion 

 of the foreign body through a temporary or permanent anus.^^ 



In Amoeba, the waste residue may leave the body at any spot in 

 the cell-surface exactly as the ingestive function may also be active at 

 any point. In fact there is diffuse monocytic egestion. 



In the higher Protozoa, the egestive function becomes more con- 

 fined to one particular point in the limiting surface and there the tem- 

 porary or permanent anus is formed. The evolution of the monocytic 

 egestive aperture in the Protozoa is parallel to that of the ingestive 

 opening or cell-mouth. 



In the Metazoa, the function of polycytic egestion is directly con- 

 nected with that of polycytic ingestion. The residue from the polycytic 

 digestion, or in the case of the sponges of the residue of food which has 

 not been monocytically ingested, leaves the body through the polycytic 

 egestive aperture or anus. This may be synonymous with the polycytic 

 mouth in the lowest forms, but where they are separate, some part of 

 the outer-layer is invaginated to form an organ of egestion, known in 

 ontogeny as the proctodaeum,so that the Metazoan alimentary canal 

 has three polycytic menibers corresponding to the three physiological 

 functions of alimentation : 



