62 THE PROTOZOA 



body (Fig. 4, st.). The belief that the stigma is the seat <of light- 

 perception receives support from the fact that in some cases it is 

 found associated with lens-like structures, which evidently serve 

 to concentrate light upon it and act as dioptric elements, as in 

 Pouchetia (Fig. 31). 



B. Endoplasmic Organs. 



The bulk of the endoplasm in proportion to that of the whole 

 body varies greatly in different Protozoa. In Flagellata, for 

 example, the protoplasmic body must be considered as consisting 

 almost entirely of endoplasm, the ectoplasm furnishing only the 

 delicate periplast and myonemes. Similarly, in motionless para- 

 sitic forms, such as the Coccidia or the ' ccelomic ' Gregarines 

 (p. 326, infra], the body within the cuticle is entirely endoplasm. 

 On the other hand, in Ciliata, in which the ectoplasm may give rise 

 to a number of different structures, the endoplasm is often a rela- 

 tively restricted region of the body. In these examples that have 

 been cited, the ectoplasm and endoplasm are probably stable 

 layers, and their relative proportions are consequently more or less 

 constant for a given phase of the life-history ; but in amoeboid forms, 

 as already pointed out, ectoplasm and endoplasm are interchange- 

 able, and the amount of each layer present hi an organism varies 

 with the extent of its body-surface ; that is to say, the proportion of 

 ectoplasm to endoplasm is greatest when the amoeba is moving 

 actively and throwing out many pseudopodia, and least when it is 

 in a resting condition and has assumed the spherical form. 



As stated above, the endoplasm is a fluid, granular substance, 

 which contains various enclosures connected with the nutritive 

 function, and also the nucleus or nuclei. Hence it may be re- 

 garded as the seat of trophic and reproductive functions. The 

 nuclear apparatus will be dealt with in a separate chapter, since it 

 belongs, strictly speaking, neither to the ectoplasm nor the endo- 

 plasm, though commonly lodged in the latter. In this chapter 

 only the structural elements connected with the function of food 

 ingestion and assimilation will be described. 



The contents of the endoplasm vary greatly, according to the 

 mode of life of the organism. In saprophytic and most parasitic 

 forms no special organs are found in connection with the nutritive 

 function, the food being simply absorbed in a soluble condition 

 at the surface of the body, probably by the aid of enzymes secreted 

 by the organism, but not by any recognizable organs. In holozoic 

 and holophytic forms, however, special organs, differing widely in 

 each case, are present for the assimilation or elaboration of food. 



1. In holozoic Protozoa the organs of assimilation take the form 

 olfood-vacuoles, minute droplets of fluid in which the solid particles 



