PROTOZOA. 131 



their interior ; they may be regarded as the earliest dawn of a circu- 

 latory system. 



The Protozoa are divided into three classes. In the first, to which 

 the name of Bhizopoda has been given, the body is composed entirely 

 of the gelatinous matter above described, motion being effected by the 

 extension of portions of the substance into filaments or processes of 

 various forms. 



The second class, including the sponges, consists entirely of asso- 

 ciated cell animals, the individual cells resembling those of the preced- 

 ing class in their power of extending the substance of their bodies 

 in all directions ; but in this class they are united by a mucilagi- 

 nous intercellular substance, and supported upon a horny framework. 

 From the masses formed by these creatures being perforated in 

 every part with minute orifices, they have received the denomination 

 of Porifera. 



The animals constituting the third class of the Protozoa have been 

 called Infusoria. They are generally solitary unicellular animals, and 

 difier from the Bhizopoda in having the outer surface of the body of a 

 somewhat firmer consistence than the rest of their substance. They 

 are usually furnished with a mouth, and their movements are effected 

 by means of cilia, or of one or more long filiform appendages attached 

 to one extremity of the body. 



In the valuable voluminous treatises on this interesting department 

 of nature, the above division would be strictly adhered to ; but as our 

 object is to give a popular account of the wonders disclosed by the 

 microscope, we will not closely follow the path of the learned investi- 

 gators of this branch of science ; and hence will avoid as much as pos- 

 sible the adoption of those terms so alarming to the uninitiated, culling, 

 as we pass, only those objects of most interest, and likely to create a 

 desire for further knowledge; whereby there will be found a future 

 pleasure in overcoming the obstacles by which the learned render so 

 mysterious and repugnant to the popular mind the stores of their 

 gathered wealth. 



AMiEBA. 



In the deposit formed at the bottom of fresh-water ponds, we may 

 often meet with a singular minute gelatinous body, which constantly 

 changes its form even under our eyes ; and moves about by means of 

 finger-like processes, which it appears to have the power of shooting 

 out from any part of its substance. This shapeless mass is well known 

 to microscopic obseiTers under the name of the Proteus {Amoeba dif- 



