THE LIVING WORLD 217 



rounded by tentacles, the whole body being made up of 

 two layers of cells, which nevertheless give signs of 

 forming most simple muscle-substance and nerve-sub- 

 stance, tissues to be more distinctly referred to shortly. 

 Another great group consists of the star-fishes, sea- 

 urchins, crinoids, &c., all of which are called Echino- 

 derms, and though essentially simple in structure, may 

 consist of a prodigious number of small juxtaposed 

 calcareous parts. 



Next may be mentioned the small animals known as 

 wheel-animalcules, and minute creatures which live in 

 compound aggregation, such as the well-known sea-mat 

 Flustra (often popularly regarded as a seaweed), which 

 minute creatures are termed Bryozoa or Polyzoa. 



A multitude of creatures are known as worms. Such 

 are the many internal parasites, with their allies, and 

 also leeches, earthworms, and aquatic worms. 



To these succeed the two great groups of (i) arthropods 

 and (2) molluscs.* Of their structure, however, we will 

 say nothing till we have first briefly described the 

 organisation and functions of one of the highest class 

 of backboned animals that to which we belong which 

 class comprises all mammals (or mammalia), so called 

 because their females give suck to their young through 

 their mammary glands and breasts. 



From this class, which includes all beasts, we will 

 select the cat for consideration, as that will well serve to 

 show how great is the difference between the most complex 

 inorganic body and a highly organised living creature. 



The external form of the cat needs no description 

 here. Beneath its skin lies the flesh, and these enclose 

 the bones skull, backbone, ribs, and limb-bones. 



* See post, pp. 234-238. 



