INVERTEBEATE ANIMALS. 



SUB-KIXGI)0:\I I. PEOTOZOA. 



CHAPTEE I. 



1. General Characters of the Protozoa. -2. Classification. 

 3. Gregarinida. 



The sub-kingdom Protozoa (Gr. protos, first ; and zofJn, an animal), 

 as the name implies, is the lowest division of the animal kingdom, 

 and its limits are therefore necessarily not yet strictly defined. The 

 Protozoa comprise an enormous number of animals, which are mostly 

 so small as to be invisible to the naked eye, and can only be satisfac- 

 torily examined under pretty high powers of the microscope. For 

 this reason, and because they are almost universally fovnid in water, 

 these cieatures, often popularly called " animalcules," are almost un- 

 known to the majority of people. The microscopical forms of the 

 Protoioa swarm in most stagnant pools, and in all waters charged 

 with organic matter so as to afford them food. Every worker with 

 the microscope is familiarly acquainted with them, and they exhibit 

 phenomena which in many cases render them objects of the highest 

 interest. From their low position in the animal scale, it arises that 

 the Vroto-jHi, are mainly characterised by the absence of organs and 

 structui-es which occur in higher beings, and they possess few posi- 

 tive characters by which they can be distinguished. 



The Proto-()<r\\\;\y be defined as aiiimah, generulhf of a very mi- 

 nute size, and composed of unmodified or slightly modi lied protoplasm. 

 The tjipie(d members of the group are " unieelhUar,'' the primitive mass 

 of protoplasm derehiping a '^nucleus,'' and thus becoming a " cell." 

 The " niiclens" may, hoirevcr, be absent ; ami it is only in the Spon/jes 



