92 PROTOZOA. 



in 1838-9, all but the simplest plants and animals are built 

 up of many unit masses of living matter or cells. The 

 Protozoa are, with few exceptions, unicellular; all other 

 animals (Metazoa) are multicellular. A Protozoon is there- 

 fore comparable to a blood-cell, a muscle-cell, or a ciliated 

 cell, — to one of the many units which make up a higher 

 animal. We may compare a rhizopod with a white blood 

 corpuscle, an infusorian with one of the ciliated cells 

 lining the windpipe, a gregarine with a degenerate muscle- 

 cell. 



But there is this difference, a single-celled Protozoon is 

 an independent organism complete in itself. Within the 

 compass of its unit mass, all the usual functions of con- 

 tractility, irritability, digestion, secretion, respiration, and 

 excretion, are discharged; while in a higher animal, in which 

 division of labour has been established, each cell is more or 

 less dependent upon its neighbours, and its functions are 

 restricted or specialised in range. A Protozoon is a cell 

 which is physiologically complete in itself. 



Moreover, it was soon realised, as a corollary of the cell 

 theory, that every Metazoon begins to develop from the 

 repeated division of an egg-cell or ovum, which has been 

 fertilised in most cases by a male-cell or spermatozoon. It 

 became evident that the Protozoa correspond to these repro- 

 ductive cells. The Protozoa- are comparable to the young 

 ova, or to the primitive-male-cells of higher animals. But the 

 primitive-male-cell of a higher animal divides into a number 

 of spermatozoa, which cling together for a while, separate, 

 and are liberated from the body, mostly to die ; while the 

 numerous cells into which a Protozoon may divide, go apart 

 and live independent lives. Or again, the young ovum of a 

 higher animal increases in size, makes a preliminary attempt 

 at division on its own account, but dies in most cases unless 

 it be united with a male-cell. When thus fertilised, it divides 

 repeatedly, and the result is an embryo of coherent cells ; 

 whereas the units into which a Protozoon divides usually 

 part company at once. In short, the Protozoa do not 

 form a "body"; "they leave off where higher animals 

 begin." 



It has been necessary to insert many saving clauses in 

 speaking of the unicellular character of Protozoa, for this 



