218 THE ANIMAL KINGDOM. 



Sub-family 3. MOLLTJSCA, having Six classes. 



ACEPHALA, or Headless. 



1. Tunicata. 



2. Brachiopoda. 



3. Lamellibraachiata. 



ENCEPHALA, Having a Head. 



4. Pteropoda. 



5. Gasteropoda. 



6. Cephalopoda. 



The Infusoria, the first class of the Eadiata, comprise 

 all those microscopic organisms called "Protozoa;" they 

 consist of a membranous cell with fluid contents, and are 

 very nearly allied to the Protophytes or lowest vegetable 

 existences. It is still a matter of discussion amongst 

 physiologists and microscopists as to which kingdom shall 

 claim certain of them, but, according to Dr. Carpenter, it 

 .appears that the vegetable cell-beings have two envelopes, 

 the inner one albuminous and the outer one cellulose or 

 .starchy, and the following passage occurs in his treatise on the 

 Microscope (which should be in the hands of all enquiring 

 minds) : 



"The animal cell, in its most complete form, is com- 

 parable 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 anything 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 primordial utricle. 

 In its young state it seems always to contain a semi-fluid 

 plasma, which is essentially the same as the protoplasm of 

 the plant, save that it does not include chlorophyll 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 sub-division, it also 

 (especially among Protozoa) may give origin to new cells by 

 the breaking up of its contents into several particles." 



Some of these creatures are so exceedingly minute that 

 hundreds of millions may be contained in a drop of water. 

 They have no organs of any kind, but consist of single cells 



