39 



PROTOZOA. 



CHAPTER I. 



1. GENERAL CHARACTERS OF THE PROTOZOA. 2. CLASSIFICATION. 

 3. GREGARINIDJ;. 



1. General Characters. The sub-kingdom Protozoa, as the 

 name implies, includes the most lowly organised members of 

 the animal kingdom. From this circumstance it is difficult, 

 if not impossible, to give an exhaustive definition, and the 

 following is, perhaps, as exact as the present state of our 

 knowledge will allow. 



The Protozoa may be defined as animals, generally of minute 

 size, composed of a nearly structureless jelly-like substance 

 (termed barcode '), showing no composition out of definite parts 

 01- segments, having no definite body-cavity, presenting no traces 

 of a nervous system, and having either no differentiated ali- 

 mentary apparatus, or but a very rudimentary one. 



The Protozoa are almost exclusively aquatic in their habits, 

 and are mostly very minute, though they sometimes form 

 colonies of considerable size. They are comprised of a more 

 or less contractile, jelly-like substance, called ' sarcode ' or 

 * animal protoplasm,' which is semi-fluid in consistence, and 

 is composed of an albuminous base with oil-globules scattered 

 through it. Granules are generally developed in the sarcode, 

 and in many cases there is a definite internal solid particle, 

 termed the 'nucleus.' 



In no Pruwzoon are any traces known of anything like 

 the nervous and vascular arrangements which are found in 

 animals of a higher grade. A nervous system is universally 

 and entirely absent, and the sole circulatory apparatus con- 

 sists in certain clear spaces called 'contractile vesicles,' which 

 are found in some species, and which doubtfully perform the 

 functions of a heart. A distinct alimentary cavity is present 

 in the higher Protozoa, Kit in many there is none, and in all 

 the digestive apparatus is of the simplest character. Organs 

 of generation, or at any rate differentiated portions of the 

 body which act as these, are sometimes present ; but in many 

 -cases true sexual reproduction has not hitherto been shown to 

 exist. 



