DEVELOPMENT. 
221 
name to any animal form capable of leading an indepen¬ 
dent life; and morphological 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 personas, 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 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 
Arthropoda.175,100 
Echinodermata. 800 
Mollusca. 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, T^inr of an inch in diameter, the smallest animal 
ever measured, and the Whale, one hundred feet long, the 
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 
