DEVELOPMENT. 221 



name to any animal form capable of leading an indepen- 

 dent life ; and morj)hological individuals, one of which is 

 the total product of an egg. Such an individual may be 

 a single physiological individual, as the Fish ; or many 

 united, as the Coral stock; or many separate physiological 

 individuals, as in the Hydroids or Plant-lice. The single 

 members of such a compound morphological individual 

 are called zooids, or personoe, and are found wherever 

 asexual reproduction takes place. 



7. Relations of Number, Size, Form, and Hank. 



The Animal Kingdom has been likened to a pyramid, 

 the species diminishing in number as tiiey ascend in the 

 scale of complexity. This is not strictly true. The num- 

 ber of living species known is at least 300,000, of which 

 more than nine tenths are Invertebrates. A late enumer- 

 ation gives the following figures for the number of de- 

 scribed species : 



Protozoa 2,700 



CcElenterata 1,560 



Vermes 5,580 



Artliropoda 175,100 



Echinoderinata 800 



Molliisca 20,210 



Vertebrata 25, 200 



These figures are lower than those usually given. Of 

 Vertebrates, Fishes are most abundant; then follow Birds, 

 Mammals, Reptiles, and Amphibians. There are usually 

 said to be about 200,000 species of Insects. 



The largest species usually belong to the higher classes. 

 The aquatic members of a group are generally larger than 

 the terrestrial, the marine than the fresh-water, and the 

 land than the aerial. The extremes of size are an Infu- 

 sorium, tbto-u ^f ^" i"^'"" '" diameter, the smallest animal 

 ever measured, and the Whale, one hundred feet long, tlie 

 largest animal ever created. The female is sometimes 

 larger than the male, as of the Nautilus, Spider, and Eagle. 

 The higher the class, the more uniform the size. Of all 



