PROTOZOA. 21 



tosoon, however, without exception, liave any traces of a nervous 

 system been hitlierto detected ; and in none, even in those which 

 possess a mouth, is tliere any distinct and definite cavity or chamber 

 within the body in which the paiticles of food are received. No 

 organs of sense (except, perhaps, occasionally veiy rudimentary ones) 

 exist in any of the Protozoa — that is to say, there are no distinct 

 organs fitted for the reception of impressions produced l>y light or 

 sound ; but the general surface of tlie body ajipears capable of re- 

 ceiving the impressions produced by contact with foreign bodies, 

 and therefore acts as an organ of touch. The power of active loco- 

 motion is enjoyed by most of the Protozoa ; but in some cases this 

 is very limited, and in other cases the animal is permanentlj' fixed 

 in the adidt condition. The apparatus of locomotion in the Protozoa 

 is of a varied nature. In many cases, especially in the higher forms, 

 movements are effected by means of little h,-di-Iike pi'ocesses, which 

 are called " cilia " (Lat. cilium, an eyelash), and which have the 

 power of vibrating or lashing to and fro with great rapidity. In 

 other cases the cilia are accompanied or i-eplaced by one or more 

 long whip-like bristles, which act in the same fashion, and are 

 known as " flagella." Amongst the lower Protozoa the mo.st char- 

 acteristic organs of locomotion are the so-called " pseudopodia " (Gr. 

 pseudos, falsity ; podes, feet) . These consist of variously shaped 

 filaments, threads, or finger-like pi'ocesses of sarcode (fig. 3), which 

 the animal can thrust out from any or every part of its Iiody. They 

 are not, however, definite and permanent oigans like the cilia, for 

 they can be produced at will, and when they aie ag.uin withdrawn 

 they simply melt into the sarcode of the liodj', and leave no traces 

 of their existence. 



The three princi].iai groujDs ot tne Protozoa are kn(jwn as the 

 Oregarinida, the IVdzopoda, and the Infusoria. The iSj-ionges will 

 be here regarded as a special divi.sion of the Animal Kingdom, to 

 which the name of Porifera may be applied. 



Class 1. Gregarinida. — The Gregarinida may be defined as 

 parasitic Protozoa which have no mouthy and hare iio pjover of giving 

 out pseudopodia. They are usually looked ujjon as forming the 

 lowest class of the Protozoa ; but in all probability nmch of their 

 degraded character, as we shall see in other cases, is due to the fact 

 that they are mostly internal yjarasites, and are therefore not de- 

 pendent on their own exeitions for food. They vary in size from 

 less than the liead of a small pin up to nearly half an inch in length, 

 when they look something like small worms ; and they are found 

 living in the interior' of various animals, es]:iecially the cockroiich 

 and the earth-worm. 



In arratomical structure a Oregarina rrsually pr-esents the appear- 

 3 



