DEVELOPMENT 



433 



Such an individual may be a single physiological indi- 

 vidual, 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 

 persona, and are found wherever asexual reproduction 

 takes place. 



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



The animal kingdom has been likened to a pyramid, 

 the species diminishing in number as they ascend in the 

 scale of complexity. This is not strictly true. The 

 number of living species known is at least 300,000, of 

 which more than nine tenths are invertebrates. A late 

 enumeration gives the following figures for the number 

 of described species : 



Protozoa 4,000 



Coslenterata 3>5 



Vermes 55^o 



Arthropoda 250,000 



Echinodermata . . . 2,300 



Mollusca 21,000 



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 and it is esti- 

 mated that there are about 500,000 living species in the 

 animal kingdom ; about 40,000 extinct species have been 

 described. 



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 Infusorium, ^Jo^ of an inch in diameter, 

 and the whale, eighty-five feet long, respectively the 

 smallest and the largest animal ever measured. The 

 DODGE'S GEN. ZOOL. 28 



