THE ORGANIZATION OF THE PROTOZOA 



33 



where a thin cuticle is present which permits changes in shape, 

 caused by the contractility of the enclosed protoplasmic body. 

 Such forms are not amoeboid, but exhibit rhythmical changes of 

 form produced by contractions of the superficial body-layer in a 

 manner somewhat recalling peristaltic movement, and are com- 

 monly said to be metabolic (Fig. 15) : and 

 since such movements are characteristic 

 of some species of the genus Euglena, 

 they are sometimes called euglenoid. 



In most cases, however, in which a 

 cuticle or firm cortex is present, a definite 

 and characteristic body-form is main- 

 tained, subject only to such changes as 

 may result from curvatures of the body, 

 or temporary shortening of its axis in a 

 particular direction, brought about by 

 the contractility of the living body. An 

 envelope of this kind, which may vary in 

 consistence from a thin, flexible cuticle 

 to a rigid inflexible cuirass, or " lorica," 

 inhibits completely the natural tendency 

 of the fluid protoplasmic body to round 

 itself off a tendency, however, which 

 frequently reasserts itself during resting 

 phases of the organism, when the cortex 

 may be softened or absorbed. Hence it 

 is very common to find that the resting 

 phases of Protozoa revert to the primi- 

 tive spherical form, whatever the shape 

 characteristic of the organism in an 

 active state. 



A close-fitting cortex or cuticle which 

 is essentially a part of the body itself 

 must be distinguished clearly from struc- 

 tures built up by the organism externally 

 to the body to afford shelter or support. 

 Such a structure is termed variously a 

 "shell," "test," or "house." The 

 formation of protective shells, into which 

 the body can be completely retracted, 



and' from which it can emerge to a greater or less extent, is of 

 extremely common occurrence amongst the naked-bodied Sarcodina. 

 The forms of these shells, their structure and mode of formation, 

 exhibit an almost infinite variety, and can only be described here in 

 a quite general manner. 



3 



FIG. 15. Astasid tenax, two 

 individuals showing the 

 changes of form due to 

 metabolic movement, oes., 

 (Esophagus ; c.r., reservoir 

 of the contractile vacuole ; 

 N., nucleus. After Stein. 



