56 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



pelled by the presence of different chemical substances 

 in the water ; and that they are sensitive to contact with 

 foreign bodies and with one another. Thus it is proved 

 that these simple organisms possess the rudiments of 

 the nervous .properties seen in the higher animals. Cer- 

 tain species contain a green coloring substance (haema- 

 tochrome) which is chemically allied to the chlorophyll 

 of plants. Others, again, pass through amoeboid stages 

 resembling similar phases in the development of some 

 of the lowest plants. Because of these resemblances 

 some of the Protozoa are almost indistinguishable from 

 the lowest members of the plant kingdom (Protophyta). 

 On account of the apparent simplicity of their struc- 

 ture, it is difficult to select features by means of which 

 the animals in this group may be classified. The diffi- 

 culty is further increased by the fact that in the course 

 of their development some forms pass through stages in 

 which they resemble other species in the same branch. 

 In every case, however, it is found that certain phases 

 of their development predominate, and these well- 

 marked phases permit of dividing the Protozoa into 

 five classes. 



CLASS i . Rhizopoda 



These are Protozoa which are predominantly amoe- 

 boid in shape and which move by means of pseudopodia, 

 as the slow-moving protrusions of the protoplasmic body 

 substance are called (Figs, i, 213). The body usually 

 contains a nucleus and a contractile vacuole. The com- 

 mon amoeba or proteus animalcule belongs in this class 

 (Fig. i). Some of the Rhizopods secrete shells of 

 chitin (Arcella), or construct a covering made of parti- 

 cles of sand (Diff.ugia). Both of these organisms are 

 found in fresh water in America. The most primitive 

 representative of the group is Protamceba, in which 



