10 THE DIFFERENCES BETWEEN ANIMALS AND PLANTS. 



are two distinct elements — namely, nerve- cells and nerve- 

 filaments — which have separate structural differences. 



Nerve-cells are found in the brain, in the spinal cord, in 

 the so-called ganglia of the lower animals, &o. ; they are really 

 central areas for the nervous stimuli. Each nerve-cell or gang- 

 lion cell possesses a very distinct nucleus and nucleolus, and 

 one, two, or more processes, when they are known as uni-, 

 bi-, or multipolar ganglion cells. One root is always that of a 

 nerve-filament. 



ISTerve-fibres are of two kinds : one variety carries impulses 

 — sensations — from the central organ (cells) to the peripheral 

 organs, — these are called motor or secretory fibres ; the other 

 carries impulses from the periphery to the central organs, and 

 are known as sensory fibres. In most cases the sensory nerves 

 are united at their peripheral end with the so-called " end- 

 organs'' in the skin, &c., tliese end-organs being derived from 

 the modified epithelial cells. 



Such are some of the modifications that are assumed by cells 

 in the animal kingdom. 



The lowest animals, we sliall see, possess neither tissues nor 

 organs composed of cells, and yet each organism, although only 

 a single cell, is complete in itself and reproduces a similar 

 species. 



The Dipperenobs between Animals and Plants. 



Living bodies are divided into two groups called "king- 

 doms," the one the Animal Kingdom, the other the Vegetable 

 Kingdom. Although there arc apparently great differences 

 between the two, yet when we come to examine the lowest 

 animal forms and compare them with the lowest vegetal forms 

 we shall observe so great a similarity that it is impossible 

 to say to which kingdom they belong. In fact, there is 

 no hard-and-fast line to be drawn between these two oro-anic 

 groups. Such lowly creatures as Volvnx are treated by 



