§ SYSTEMATI€ ZOOLOGY. 
communicating the sensation of sight, one for 
sound, one for taste, and one for smell. Some of 
the other divisions of animals have more or less 
of these organs in greater or less perfection. 
All animals seem to require a system of organs 
for digesting food, for circulating fluids through 
the body, and for oxydating those fluids. In ver- 
tebral animals these organs consist of stomachs, 
to which an alimentary canal is attached ; a heart 
which is alternately compressed and expanded, 
to which a set of arteries and veins are attached— 
the former for conveying the fluids from the heart, 
and the latter for returning them to the heart; lungs 
or gills in which the same fluids are presented to the 
oxygen of the atmosphere. Some animals have 
their fluids oxydated by means of spiracles, some 
by means of exterior membranous organs, 
SYSTEMATIC ZOOLOGY. 
Animals are distributed into groups, for the 
convenience of ascertaining their names and of 
studying their affinities. bese groupes or as- 
gemblages are called Classes, Orders, Genera, 
Species and Varicties. | 
Liyneus distributed all animals into six classes, 
4, Mammalia, viviparous and suckling the young, 
2. Aves, oviparous, having two wings and two 
feet. 8. Amphibia, lungs adapted to long sus- 
pended respiration. 4. Pisces, fins and gills sub- 
stituted for lungs. 5. Insecta, antenne in most 
cases, members articulated to an external crust. 
6. Vermes, body soft, members not articulated, or 
wanting. 
Cuvier subdivided several of these classes.— 
Mammalia, ives, Amphibia and Pisces remain as 
