PKOTOPLASTA— THE LOBOSE PKOTOPLASTS. 25 



indeed in the same species under different circumstances they may be either 

 bhmt or pointed. 



The endosarc is essentially granular ; that is to say, it consists of a 

 motile hyaline protoplasm, not differentiated from the ectosarc, except in 

 its less consistence and in the thorough diffusion of conspicuous granular 

 elements. These occur in all gradations of size, from those which are 

 imiAeasurably fine and indistinct up to the largest granules which are 

 more or less darkly defined, and resemble oil-molecules. Besides the more 

 generally diffused granular constituents,, the endosarc contains variable pro- 

 portions of larger spherical corpuscles, clear or finely granular, and mostly 

 colorless or pale yellowish, but in some species usually of a bright green 

 color, and apparently of the nature of chlorophyl. 



An important element of the endosarc is the nucleus, a comparatively 

 large spherical or compressed spherical corpuscle, colorless, and mostly clear, 

 but sometimes finely granular, or more distinctly coarsely and uniformly 

 granular. In the naked forms of the Lobosa, while in motion, the nucleus 

 usually occupies a position at the posterior part of the body, or at least is 

 placed back of the centre In most of the shell-covered forms it occupies 

 a corresponding position ; that is to say, in the part most remote from the 

 mouth of the shell, — in the fundus or upper part of the body. In some 

 forms, as in Arcella, there are two or more nuclei occupying a position on 

 each side of the fundus. 



Another element of the endosarc, usually situated at its confines and 

 encroaching on the ectosarc, is the contractile or pulsating vesicle. This 

 appears as a clear, colorless, or pale roseate sphere, which is observed' very 

 slowly to enlarge, then rather abruptly to collapse and for the moment dis- 

 appear, again to reappear, commonly in the same position. The successive 

 movements of the vesicle occur with a certain degree of regularity, or 

 rhythm, whence the name of pulsating vesicle. It appears to be due to a 

 gradual concentration of water from all parts of the sarcode mass of the 

 body, forming a drop, which when it reaches a certain size excites con- 

 traction and is expelled. The phenomenon is remarkable, and is in a 

 measure subservient to a respiratory as well as to an excretory function. 



In the naked Lobosa, the contractile vesicle usually occupies a position 

 posterior to the nucleus or at the back of the body near the surface: In 

 the shell-covered kinds of flask- or vase-Hke shape, there are commonly 



