STRUCTURE OF PROTOZOA. 



135 



polypes ; and new branches, are formed either by the sub- 

 division of the polypes, or by gemmation from the connecting 

 substance. 



128. When we pass from Zoophytes to animals of still 

 simpler organization, we lose all trace of definite symmetry, and 

 find ourselves amid forms which cannot be referred to any 

 particular plan of growth. These, moreover, are for the most 

 part distinguished by an extreme simplicity of structure ; no 

 such differentiation of parts exhibiting itself among them, as 

 is shown in the " organs " of even the simplest Zoophyte or 

 Worm. Hence they are appropriately designated PROTOZOA. 

 They may, in fact, be considered as essentially consisting of 

 homogeneous particles of a jelly-like substance, to which the 

 name of Sarcode has been given ; and the chief modification 

 this undergoes, consists in the consolidation of certain parts 

 of it by the deposit of horny, calcareous, or siliceous matter, 

 so as to form a skeleton. This may take place on the outer 

 surface only, so as to form shells very like those of Mollusks 

 in miniature, as we see among Foraminifera (fig. 78); or it 

 may occur in the midst of the fleshy substance, so as to 

 form an internal network, such as presents itself in the 

 Sponge. The endowments of the " sarcode " are very extra- 

 ordinary ; and will be best understood by observation of the 

 life-history of one of those simplest Protozoa, in which the 

 whole body consists of but a minute particle of it. 



>. 77. RHIZOPODA : A, Amoeba; B, Actinophrys. 



129. Such an example is afforded by the Amoeba (fig. 77 A), 

 a creature frequently to be met with in great abundance 

 in fresh and stagnant waters, vegetable infusions, &c. Its 



