4 INTRODUCTION TO ZOOLOGY. 



the outline of the animal is so different. In others, it wiil be 

 found, not in the general aspect of the body, but in the 

 radiated arrangement of the parts surrounding the mouth. 

 Wherever, throughout this division of the animal kingdom, 

 we are able to trace in the body the existence of a nervous 

 system, it partakes of that radiated appearance which, hi 

 some species, is presented by the external figure. Some 

 creatures, in which no nervous system has as yet been dis- 

 covered, are included in this division; and as our knowledge 

 of their structure and habits is increased, our present classifi- 

 cation must be revised, and perhaps amended. 



The Kadiated animals may be treated of under four* pri- 

 mary divisions or "classes," in each of which there are found 

 animals of a higher and a lower grade of organization, viz. : 



Infusoria, or Infusory Animalcules ; 

 Entozoa, or Internal Parasites; 

 Zoophyta, or Polypes ; 

 Madiaria, or Rayed Animals. 



CLASS INFUSORIA, OR INFUSORY ANIMALCULES. 



" Whore the pool 



Stands mantled o'er with green, invisible 

 Amid the floating verdure millions stray." THOMSON. 



IF any vegetable substance be allowed to remain for about ten 

 days in a glass of water, exposed in a window to the rays of 

 the sun, the water will appear to the naked eye to have 

 undergone little change. But if a drop be taken from the 

 surface and placed under the microscope, it will exhibit such 

 a multitude of living beings swimming about, that the spec- 

 tacle cannot be looked upon for the first time without surprise, 

 and even astonishment. Nor is the feeling of wonder dimi- 

 nished when we endeavour to calculate their size, and form 

 some estimate of their numbers. If a drop of the water 



* The sponges (Porifera) are omitted, as naturalists are not yet 

 agreed as to their true nature. 



