AMONG SIMPLE SAECODE OB&ANISMS. 265 



occupy a definite position and are tlien also definite in number. 

 They mostly appear and disappear at intervals ; and in those 

 vacuoles which have a definite number and fixed position the ap- 

 pearance and disappearance follow one another at equal intervals, 

 having thus a regularly rhythmical sequence. It is not .easy, how- 

 ever, by any hard and fast line to separate these two classes of 

 . vacuoles from one another ; even those with indefinite position 

 and number sometimes show a rhythmical contraction, while they 

 all pass by intermediate conditions into those irregular liquid- 

 holding spaces so obvious in the protoplasm of plant-cells. It is 

 these conditions which have induced Hertwig and Lesser, in 

 opposition to the views of other zoologists, to assign little or no 

 systematic value to the contractility of the vacuoles. 



As may be expected in organisms of such extremely simple 

 structure and with the functions of nuti'ition and irritability 

 showing such little tendency to specialization, there is a corre- 

 sponding simplicity in the function of reproduction. This, indeed, 

 is probably limited to a simple division of the body referable to 

 the established laws of cell-multiplication ; for the assumption 

 that the nucleus exercises a sexual function, though insisted on 

 by some observers,' does not rest on a sufficient number of conti- 

 nuous and connected observations. 



In many cases, however, an encysting process becomes intro- 

 du.ced into this simple form of reproduction. The organism with- 

 draws its pseudopodia, secretes around it a membranous cyst, and 

 passes into a resting state. Within the cyst the protoplasm di- 

 vides into two or more portions, and these finally break through 

 the walls of the cyst and become free. 



Hertwig and Lesser regard this intercalation of an encysting 

 process into the development-cycle of the organism as conditioned 

 by the laws of adaptation with inheritance. Many observed phe- 

 nomena tend to show that the encysting at first arose from an 

 adaptation to external conditions, and only at a subsequent period 

 became subservient to reproduction. They suggest that it may 

 have served originally either in maintaining the vitality of the 

 organism during the drying up of the pools of water inhabited by 

 it, or in affording a protection from its enemies when, after abun- 

 dant ingestion of nutriment, it passes for the purposes of diges- 

 tion into a quiescent state. 



The organisms distinguished by the characters here enumerated, 

 together with certain marine forms — the so-called Foraminifera — 

 to whicli the present review is not intended to extend, correspond 



