CHAPTER I. 



PROTOZOA.* 



Animals in which there is one nucleus, or, if more than one nucleus, 

 in which the nuclei are disposed apparently irregularly and without 

 relation to the functional tissues of the animal. Conjugating cells 

 of the form of ova and spermatozoa are never formed. 



Structurally the Protozoa are so simple that the reproduction of 

 the species is effected either by division of the body into two or 

 more parts, or by a separation oft' of a small portion, which so nearly 

 resembles the parent in structure that the phenomenon of embryonic 

 development is almost, if not completely, absent from the life-history. 



The body is always composed of a contractile granular substance, 

 filled with vacuoles ; it may also contain a pulsating vacuole, and 

 present the phenomenon of granule currents. The pulsating vacuole 

 consists of a space without differentiated Avails filled with a clear 

 fluid. This space apparently diminishes and disappears through the 

 contraction of the surrounding plasma, and then reappears. 



There are, however, differentiations, both in the interior of the 

 body and in its external boundary, on which a classification may be 

 founded. In the simplest cases, the entire body consists of a small 

 lump of protoplasm (or sarcode, as it was at first called), the 

 contractility of which is confined by no firm external membrane. 

 This lump of protoplasm is sometimes semi-fluid, and protrudes and 

 retracts processes. It is sometimes of tougher consistence in parts, 

 and protrudes thread-like rays (Rhizopodci). Nutrition takes place 

 through the intussusception of extraneous bodies, which can be 

 surrounded and enclosed by the protoplasmic substance at any 

 portion whatsoever of the periphery of the body. In other cases the 

 body which sends out slender processes (pseudopodia) secretes silicious 

 or calcareous needles, lattice-work shells, or shells perforated by 

 holes, to shelter and protect the body (Foraminifera, Radiolaria). 



* 0. Biitschli, "Protozoa" in Bronn's Thierreich, 1880-2. 



B 



