400 r ON THE STRUCTURE AND HABITS 



Kingdom by a tribe of beings which have generally been considered as the simplest 

 form of animals.* 



If, at this season of the year, we have recourse to any pond, brook, or exposed 

 piece of water, and take but a single drop of their contents, we shall find by the 

 aid of the microscope, though invisible to the naked eye, we have secured a 

 little world full of life and motion teeming with inhabitants, endued with 

 instincts and functions as calculated to afford happiness as those we see possessed 

 by the more complicated tribes of beings that cover the surface of the immense 

 globe we live upont, 



* In the following observations I have adopted in some measure the arrangement of Mr. Kirbv 

 in his Bridgewater Treatise, as being the most popular work on the subject ; but since this differs 

 considerably from the classifications of recent writers, I have drawn up a list of the classes with 

 their sub-kingdoms, as far as I intend to illustrate them, and given a familiar example of each, 

 according to the views of Dr. Grant, Professor of Zoology and Comparative Anatomy in the 

 London University : — 



Sub-kingdom I.— Radiated or Cyclo-neurose Classes. 



These sub-kingdoms are founded on the different arrangements of the nervous system in the 

 various classes which they comprehend. In the first the nervous matter is either diffused through- 

 out the whole animal in a molecular form, or in a circular form, as Fig. 6. In the second the 

 nervous matter is arranged in two parallel lines as in Fig. 5. In the third the general arrange- 

 ment of the nervous system is circular, but its continuity is interrupted by little swellings called 

 ganglions, as in Fig. 7. 



f We presume Mr. Lankester here intends merely to state that every animal, however low in 

 the scale, is so organised as to possess a certain degree of happiness, and to prevent its being 



