22 GENERAL ORGANIZATION OF THE PROTOZOA 



the more fluid alveolar substance, while the specific kinetic elements, 

 if present, are concentrated. Such an hypothesis might very well 

 account for the contractility of the ectoplasm of an ameba or for the 

 various locomotor appendages of flagellated and ciliated forms (see 

 page 29). 



It is on the basis of these protoplasmic modifications that the pro- 

 tozoa are grouped into classes, orders, and finer subdivisions, and the 

 most important of these have to do with the changes undergone by the 

 outer protoplasm. This is the part of the cell that comes in contact 

 with the surrounding medium, and this is the part, therefore, if any, 

 which becomes changed by such contact. Being on the outside, it is 

 the region of the cell for food ingestion, and we find it differentiated 

 into mouth parts and into protoplasmic modifications for the procuring 

 and directing of food. It is also the seat of motion, and may be 

 differentiated into a great variety of motile organs which are so char- 

 acteristic that classification is based mainly upon them. These motile 

 organs, all of which may be traced back to a similar primitive type, may 

 become modified into complex organs of the cells, while the function 

 of locomotion is frequently changed into that of food getting, or into 

 a sensory function of touch. It is an interesting point in this connec- 

 tion that the sensory apparatus arises in the outer or cortical plasm as 

 a response of protoplasm to the surrounding medium, and it is signifi- 

 cant that in all higher animals the sensory and nervous systems arise 

 from the outermost layer of cells, the ectoderm. 



In many protozoa, especially among the simpler rhizopods and 

 some of the sporozoa, there may be no distinction between the inner 

 and the outer protoplasm. Such cases, however, are exceptional, for 

 in the majority of protozoa a well-marked ectoplasm can be distin- 

 guished. In most cases the difference appears to be mainly in the 

 presence or absence of granules, their distribution depending upon 

 the density of the plasm. No great morphological value can be placed 

 upon this regional difference, for it appears to be only an index of the 

 physical condition of the protoplasm. In Ameba proteus, for example, 

 the outer layer is dense and the granules of the alveoli are forced into 

 the more fluid endoplasm, but in pelomyxa the protoplasm appears to 

 be everywhere the same in density and the granules penetrate to the 

 very periphery. In some of the rhizopods, especially the shelled forms, 

 the distribution of granules according to density is so marked that 

 several zones can be made out. In this connection it is significant 

 that in the artificial mixtures which Biitschli so successfully made to 

 imitate protoplasm, a similar regional differentiation into outer and 

 inner structures could be distinguished, a result due in this case to 

 surface tension. 



(6) Membranes, Shells, and Tests. It is possibly due to such a 

 tendency of protoplasm to stiffen under the influence of surface tension 



