191 



protective envelope is possible, whilst in the latter a more or less per- 

 manent mouth and alimentary area are essential. 



Taking, then, this necessity for solid nutrition in animals as the 

 basis of their morphological differentiation, we can conveniently 

 divide the processes by which an animal maintains its protoplasmic 

 energy into three groups: namely. 



1) Ingestive processes. 2) Digestive processes. 3) Egesti ve pro- 

 cesses, and correlative to an elaboration of these we have the morpho- 

 logical differentiation of the three sets of organs : Ingestive, Digestive 

 and Egestive organs. 



Organs which occur as the result of the differentiation of a single 

 cell, and functions conducted by cells either singly, or independently 

 when the cells are in a mass, may be termed monocytic (in contra- 

 distinction to the form and function of tissues, which thus may be 

 termed polycytic). The cell-mouth^, cell-anus, and cell-digestive 

 tract are thus monocytic mouth, etc., whereas the mouth of Hydra 

 may be termed a polycytic ingestive aperture. Similarily, the digestion 

 of food inside a cell, «intra- cellular« digestion, may be conveniently 

 and comprehensively termed monocytic digestion, whereas »extra- 

 cellular« digestion may also be known as polycytic digestion. Mono- 

 cytic organs only are found in the Protozoa, whereas both occur in 

 the Metazoa. 



Under »Ingestion« are included all those organs which assist towards 

 the ingestion of solid food. In the narrower sense this will mean aper- 

 tures leading into the digestive area i. e. »mouths«, whatever their 

 morphological value, but in the wider sense, it will comprise various 

 organs developed in connection with the »mouth«, including such di- 

 verse structures as cilia, flagella, pseudopodia, tentacles, jaws, etc. 



In a form like Amoeba, we find that it is quite impossible to 

 distinguish between the functions of locomotion and ingestion, both 

 are in the diffuse condition, and the ordinary contractile activity of 

 the protoplasm serves for the ingestion of solid particles. In higher 

 Protozoa illustrated by the Ciliata, there is a subservience of certain 

 of the diffused locomotor organs to the special locomotor function of 

 ingestion 2 in accordance with the location of the ingestive function 

 to one particular organ the mouth. It follows from this, that the organ 

 of ingestion being primitively a specialised part of the general loco- 

 motor system, must belong phylogenetically to the outer layer, the 



1 E. R. Lankester, Encyclop. Brit. »Protozoa«, 1885. 



" A like development of ingestive organs from locomotor organs can be seen 

 in the history of mouth-appendages in Arthropode. 



