POLYGASTRIA. 



17 



very existence we are indebted to the microscope, are by no means 

 the least complex of organised beings ; they belong, in fact, to the 

 higher division of organic nature, and manifest the distinctive pro- 

 perties of animals in the most striking and unequivocal manner. 



If you skim a small portion of the green matter, which in summer 

 time mantles the surface of a stagnant pool, place a drop of this in 

 the object-holder of a microscope, and examine it with a glass of a 

 quarter of inch focus, you will find it teeming with animal life ; you 

 will see numerous little objects, of one or other of the forms, depicted 

 in this diagram, darting with rapidity across the field of view, or 

 gyrating or revolving on their axes. If you examine in like manner 

 a drop of water in which has been infused any vegetable or animal 

 substance, and which contains the particles of such substances in a 

 state of decay or decomposition, you will find such infusions similarly 

 tenanted with these active animalcules ; they have been termed from 

 this easy and common mode of procuring them, the animals of in- 

 fusions, or Infusoria. 



The earlier microscopical observers confounded all the minute 

 living objects which they thus met with under that term ; but the 

 progressively increasing pains and discrimination of later observers 

 have removed the embryos of polypes, worms, and insects from this 

 motley and heterogeneous group, and have restricted it to those ?ini- 

 mals which, in their fully developed states, manifest a form of body 

 devoid of radiated arms or tentacles, more or less amorphous, without 

 definite locomotive members, moving by means of minute superficial 

 vibratile cilia more or less diff'used over the surface of the body, or 

 aggregated in circular groups near the head, where they produce by 

 their successive action the appearance of ra- 

 pidly rotating wheels. It is always the largest 

 species of Infusoria which are provided with 

 the last specified arrangement of vibratile 

 cilia, and these " wheel-animalcules," as they 

 are termed, being endowed with a higher 

 type of organisation, more especially of the 

 digestive system, constitute a distinct class of 

 Infusorial Animalcules, to which I shall refer, 

 after first noticing the anatomical characters 

 of the lower organised and more diffusely 

 ciliated group {fig. 5.). 



The species of this group possess numerous 



clear globular sacs in their interior, which 



rajjidly receive coloured nutriment, when in 



a sufficiently subdivided state : these sacs are 



c 



