CHAPTER IX. 



MICROSCOPIC FORMS OF ANIMAL LIFE: PROTOZOA; ANIMALCULES. 



259. Protozoa. Passing on, now, to the Animal Kingdom, we 

 begin by directing our attention to those minute and simple 

 forms, which correspond, in the Animal series, with the Proto- 

 phyta in the Vegetable (Chap. VI) ; and this is the more desira- 

 ble, since the formation of a distinct group, to which the name 

 of Protozoa (first proposed by Siebold) may be appropriately 

 given, is not merely one of the most interesting results of recent 

 Microscopic inquiry, but is a subject on which it is particularly 

 important that the Microscopic observer should know what the 

 Physiologist believes himself to have ascertained. This group, 

 which must be placed at the very base of the animal scale, 

 beneath the great subkingdoms marked out by Cuvier, is charac- 

 terized by the extreme simplicity that prevails in the structure 

 of the beings composing it ; these being either isolated cells, or 

 aggregations of cells wherein no such differentiation of parts 

 exhibits itself, as constitutes the " organs" of even the simplest 

 Zoophyte or Worm. "We have in the first place to consider, 

 therefore, what are the essential characters of the Animal cell ; 

 and what are the precise relations of the Protozoa to the Proto- 

 phyta, to which they seem to bear so close an affinity. 



260. The Animal cell, in its most complete form, is compara- 

 ble in most parts of its structure to that of the Plant ; but differs 

 from it in the entire absence of the " cellulose-wall," or of any- 

 thing that represents it, the cell contents being enclosed in only 

 a single limitary membrane, the chemical composition of which 

 (being albuminous) indicates its correspondence with the primor- 

 dial utricle ( 147). In its young state, it seems always to con- 

 tain a semi-fluid plasma, which is essentially the same as the 

 "protoplasm" of the Plant, save that it does not include chloro- 

 phyll-granules ; and this may either continue to occupy its cavity 

 (which is the case in cells whose entire energy is directed to 

 growth and multiplication), or may give place, either wholly or 

 in part, to the special product which it may be the function of 

 the cell to prepare. Like the Vegetable cell, that of Animals 

 very commonly multiplies by duplicative subdivision ; and it 

 also (especially among Protozoa) may give origin 'to new cells, 



