PROTOZOA 135 



CLASS I. SARCODINA 



The Sarcodina are naked cells (Fig. 62, A-D). Many 

 have shells secreted around their little bodies, but such 

 accretions are not actually a part of the protoplasm. The 

 protoplasm of these animals is, then, without much speciali- 

 zation, but it may form elaborate structures on the outside. 

 The amoeba will be considered as a representative of the 

 Sarcodina. 



THE AMCEBA 



Self-maintenance. There are a number of different 

 kinds of amoebas and their structure and habitats vary 

 considerably. The species chosen for the subject of this 

 discussion, Amoeba proteus (Fig. 62, A), is found in water, in 

 the brown coating of microscopic plants such as accumulates 

 in water troughs and among aquatic vegetation. Its food 

 consists of minute plants and animals which are engulfed 

 by the soft body. There is no mouth or definite place for 

 the entrance of food. The body consists, like all cells, of a 

 central nucleus surrounded by cytoplasm. The cytoplasm 

 is of about the consistency of the white of an egg, and shows 

 a differentiation into two regions: an outer denser portion, 

 the ectosarc, and an inner granular portion, the endosarc. 

 When an amoeba engulfs its food the ectosarc is first turned 

 in to form a little pocket, then surrounds the food com- 

 pletely. The obj ect eaten goes into the endosarc, surrounded 

 by a little water and the ectosarc which came in as it was 

 being engulfed. The watery secretion formed around the 

 particle makes a "food vacuole," and contains certain sub- 

 stances which dissolve the food so that it may be gradually 

 absorbed by the surrounding endosarc. If there are sub- 

 stances in the food which cannot be digested and assimi- 

 lated, they are dropped out from the body anywhere, as the 

 amoeba moves along. In spite of the extreme simplicity of 

 the amoeba's body, it is able to discriminate between objects 

 fit for food and those which are unsuitable. It rejects sand 



