PROTOZOA. 21 



tozoon, however, without exception, have any traces of a nervous 

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

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

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

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

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

 organs fitted for the reception of impressions produced by light or 

 sound ; but the general surface of the body appears 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 permanently fixed 

 in the adult 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 hair-like processes, which 

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

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

 other cases tlie cilia are accompanied or replaced by one or more 

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

 known as " flagella." Amongst the lower Protozoa the most char- 

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

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

 filaments, threads, or finger-like processes of sarcode (fig. 3), which 

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

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

 they can be produced at will, and when they are again withdrawn 

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

 of their existence. 



The three principal groups ot tne Protozoa are known as the 

 Oreqari7uda, the Rhizopoda, and the InfusoriK. The Sponges will 

 be here regarded as a special division of the Animal Kingdom, to 

 which the name of Porifera may be applied. 



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

 parasitic Protozoa which have no mouth, and have no pou-er of giving 

 out pseudopodia. They are usually looked upon as forming the 

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

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

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

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

 less than the head 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, especially the cockroach 

 and the earth-worm. 



In anatomical structure a Gregarina usually presents the appear- 



