38 THE PROTOZOA 



2. The Ectoplasm. 



In many Protozoa, especially among the Rhizopoda, there may be 

 no distinction between ectoplasm and endoplasm. These cases, how- 

 ever, are exceptions, for in the majority of forms a well-marked ecto- 

 plasm can be distinguished. In many cases the difference appears 

 to be only in the presence or absence of granules, and their distribu- 

 tion depends upon the density of the fluid plasm. No great mor- 

 phological importance can be attached to this regional difference, for 

 it appears to be only an index of the physical conditions of the 

 protoplasm. The body of the common rhizopod Amceba, for example, 

 consists of a more or less fluid mass in which lie suspended the 

 various granules, vacuoles, nuclei, crystals, and food particles, and, 

 as Griiber ('84) pointed out, if the plasm is thin, i.e. more fluid, the 

 contents can spread easily through the whole mass, while if the 

 plasm is dense and viscous, they will be held back by the resistance, 

 and a relatively broad ectoplasm may result. The more fluid condi- 

 tion is seen in rhizopods like Protomyxa and Pelomyxa, the denser in 

 Amceba proteus, and the majority of fresh-water shell-bearing forms. 

 In some of the latter and in a few Infusoria the distribution according 

 to density is so marked that several regions can be made out. Thus 

 Penard ('90) described no less than four zones in the shelled rhizopod, 

 Englypha, while in many Infusoria and in some Sporozoa a mem- 

 brane, ectoplasm, cortical plasm, and endoplasm, differing from one 

 another in density, can be distinguished. It is an interesting fact 

 that in the artificial mixtures which Btitschli has so successfully 

 made to imitate protoplasm, a similar regional differentiation, at least 

 as far as ectoplasm and endoplasm are concerned, may be seen. 



It is perhaps to a tendency of protoplasm to stiffen while in con- 

 tact with water that we can turn for an explanation, first pointed out 

 by Griiber ('81), of the outer condensation of protoplasm resulting in 

 the numerous membranes and tests of the Rhizopoda, and of the 

 outer coverings of Protozoa in general. The simplest form of mem- 

 brane is an almost invisible cuticle of extreme delicacy {pellicula of 

 R. S. Bergh) as in the rhizopod Amceba proteus (Griiber, '81). In 

 other forms of the same genus, however, the outer zone becomes 

 greatly thickened (A. tcntaculata, A. actinophora, Fig. 12, A), and a 

 more or less lifeless membrane results. In these thick-skinned forms 

 the membrane is often perforated by the pseudopodia, which form long 

 finger-like processes, and when retracted leave minute holes in the 

 membrane. In these cases there is usually a sharp distinction 

 between the inner plasm and the cortical part, but in many Infusoria 

 and Sporozoa there is a gradual increase in density from within 

 outward, and the outside is covered by living membranes which may- 

 become complicated by the addition of muscular fibrils {inyoncmcs), of 



