62 -ERIAL FAUNA. 
Vertebrata and Arthropoda stand pre-eminent. In the first 
we find the lowest class (/.e. fishes) is aquatic and mainly 
marine; the amphibians are freshwater and terrestrial. The 
reptiles still cling to the aquatic life but the majority 
of birds and mammals are typically terrestrial or zerial. 
In the Arthropoda, again, the Crustacea are typically 
aquatic and are in many respects the lowest class, but the 
Arachnida, and above all the Zusecta, are large and important 
terrestrial classes. Other terrestrial animals belong to the 
Mollusca (Gastropoda), Platyhelminthes, Nematoda and 
Annelida. With the exception of these three lowest phyla, 
all show special air-breathing respiratory organs, moisture is 
supplied to the food by salivary glands, and iron replaces 
copper in the blood. All are plano-symmetric (with very 
few exceptions). 
We may distinguish several subsidiary divisions :— 
1. Cursorial (running). 3. Arboreal (tree-dwelling). 
2. Fossorial (burrowing). 4. Reptant (creeping). 
3. Afrial Fauna.—This fauna is still more select and 
smaller in numbers than the last. Nearly all the birds, a 
few mammals, one or two fishes, some extinct reptiles, and 
any number of insects make up the group, They are mainly 
characterised by extremely active bodies, paired “ wings” 
as locomotor crgans, and highly-developed sense-organs. 
They resort to the terrestrial or aquatic habitat for their 
reproduction and they have themselves been derived from 
terrestrial ancestors. (In one or two cases from aquatic.) 
SUMMARISING, we may distinguish in the physical 
distribution of animals certain habitats which involve special 
physical conditions and are inhabited by special faunas. 
Of these we can clearly distinguish— 
1. Pelagic. 4. Freshwater. 
2. Neritic. 5. Terrestrial. 
3. Abysmal. 6. Azrial. 
There is also evidence for believing that the general 
trend of evolutional progress, the phylogenetic migration of 
the animal kingdom, has teen from Pelagic to Neritic, from 
Neritic to the Abysmal and the Freshwater. From the 
Freshwater it has passed to the Terrestrial and thence to 
