INTRODUCTION. 



GENERAL CHARACTERISTICS OF RHIZOPODS. 



Rhizopods consist, essentially, of a soft mass of clear or 

 granular protoplasm, usually colorless, with one or more 

 nuclei and contractile vesicles. By far, the greater num- 

 ber are provided with a shell-covering, sometimes chit- 

 inous but frequently composed of grains of sand and 

 other extraneous matters cemented together. 



A distinction is made between the outer protoplasmic 

 layer and the interior mass of the body ; the outer layer 

 is called the ectosarc, the inner the endosarc. The dis- 

 tinction between endosarc and ectosarc is not always 

 clear. There is no definite line of demarkation between 

 them, such as would make the former correspond to a 

 cell-wall. Dr. Wallich regards them as only temporarily 

 distinct portions of sarcode, mutually convertible into one 

 another. The ectosarc is merely a more or less thick 

 layer of protoplasm, which seems to have undergone 

 some change by contact with the surrounding medium. 

 The endosarc is the interior protoplasmic mass; it is 

 usually more coarsely granular than the ectosarc. Within 

 the endosarc, the nuclei and the pulsating vesicles are 

 found. The former are usually large, spherical, hyaline 

 corpuscles, in most cases situated back of the middle of 

 the animal. The pulsating or contractile vesicle is a 

 colorless, or roseate, spherical structure, which is quite 



