PROTOZOA 2 1 



— a peripheral layer, the ectosare or ectoplasm, which is 

 clear and homogeneous, and an inner granular portion, the ento- 

 sarc or entoplasm. In this entosarc lie the nucleus or nuclei, 

 numerous vacuoles containing fluids or food particles, together 

 with pigment granules, minute crystals, etc. In the fresh-water 

 species there is generally in the ectosare a large vacuole, which 

 disappears and reappears at more or less definite intervals of 

 time and is therefore known as the contractile or pulsating 

 vacuole. It appears to form in the entosarc and then move 

 into the ectosare. In most of the Rhizopoda there is some 

 kind of a supporting structure or skeleton. This may be 

 gelatinous, or chitinous, or composed of particles of sand or the 

 shells of diatoms or other foreign bodies cemented together, or 

 consist of more or less elaborate structures impregnated with 

 calcium carbonate or with silica ; we may best consider the de- 

 tails of these skeletal structures in connection with the several 

 orders into which this class is divided. 



Locomotion in the Rhizopoda is effected by means of their 

 pseudopodia (Fig. 2), which enable them to move about with a 

 sort of creeping motion ; this is frequently spoken of as amoeboid 

 motion, the name being derived from Amoeba, one of the simplest 

 of the Rhizopoda. When the animal shows a differentiation into 

 ectosare and entosarc, the ectosare takes the initiative in the for- 

 mation of a pseudopodium by extending out from the surface as 

 a threadlike or fingerlike process ; into the axis of this process 

 the entosarc passes with a streaming motion. The extension or 

 retraction of the pseudopodia is due to stimuli produced by various 

 agents, such as chemical substances, food particles, mechanical 

 irritation. Often if one pseudopodium is irritated, not only 

 does that one contract, but all the others as well, so that the 

 stimulus must be transmitted through the protoplasmic substance 

 of the body of the animal from one point to another. There is 

 no definite mouth in the Rhizopoda, and hence the ingestion, or 

 taking in of food, may occur at any point on the surface of the 

 body. This is effected by means of the pseudopodia ; these 

 creep around the food particle, which is generally a small plant, 

 until the\ r completely inclose it, together with a small quantity 

 of water, and thus a food vacuole is formed. There digestion 

 takes place, and the undigested parts are given off at an}- point 



