GENERAL MORPHOLOGY 



23 



in water that we may turn for an explanation, first pointed out by 

 Gruber ('81), of the outer condensation of protoplasm resulting in the 

 numerous types of membranes and tests of the rhizopods or of the outer 

 coverings of the protozoa in general. The simplest form of membrane 

 is an almost invisible cuticle of extreme delicacy, and it would be 

 difficult to say whether such coverings are due to the physical change 

 of the protoplasm or to secretion of a covering material which gradu- 

 ally hardens in the water (as cysts are formed). In the ordinary forms 

 of ameba, at any rate, the pellicula is merely a hardening or condensa- 



FiG. 5 



A, Englypha alveolata; B, Cochliopodium, 



tion of the outer zone, and in the different species of ameba all grades 

 may be distinguished up to the relatively thick membranes of Ameba 

 tentaculata or Ameba actinophora. In other forms of protozoa there is 

 a gradual increase in density from within outward and the body of the 

 cell is covered by a living membrane which may become complicated 

 by the addition of muscular fibrils (myonemes), sensory or tactile 

 organs (cirri), or various protective structures like hooks, spines, and 

 tentacles (Figs. 5 and 6). 



Like many of the cells which constitute the tissues of higher animals, 



