192 



ELEMENTS OF SCIENCE 



FIG. 27. 



another of star-fishes and their congeners. There is yet 

 another of zoophytes, or polyps, and another of sponges, 

 and, finally, we have a sub-kingdom of minute creatures, 

 or animalculce, of very varied forms, which may make up 

 the sub-kingdom of Protozoa, consisting of animals which 

 are mostly unicellular. 



Multitudinous and varied as are the creatures which 

 compose this immense organic world, 

 they nevertheless exhibit a very re- 

 markable uniformity of composition 

 in their essential structure. Every 

 living creature, from a man to a mush- 

 room, or even to the smallest animal- 

 cule or unicellular plant, is always 

 partly fluid, but never entirely so. 

 Every living creature also consists 

 in part (and that part is the most 

 actively living part) of a soft, vis- 

 cid, transparent, colourless substance, 

 termed protoplasm, which can be 

 resolved into the four elements,* oxy- 

 gen, hydrogen, nitrogen and carbon. 

 Besides these four elements, living 

 organisms commonly contain sulphur, 

 phosphorus, chlorine, potassium, so- 

 dium, calcium, magnesium and iron. 

 In the fact that living creatures always consist of 

 the four elements, oxygen, hydrogen, nitrogen and 

 carbon, we have a fundamental character, whereby the 

 organic and inorganic (or non-living) worlds are to be 

 distinguished. For, as we have seen, inorganic bodies, 

 instead of being thus uniformly constituted, may consist 



A TUNICATE 



(Ascidia). 



* See ante, pp. 138-140. 



