INTRODUCTION. XI 
bottom of the water. The bodies of these animals are usually 
covered with scales, which keep them from injury by the pressure 
of the water. 
The fishes were divided by Linnaeus into six Orders. 
ORDERS OF FISHES. 
I. APODAL. Having bony gills ; and no ventral fins, as the eel. 
II. JUGULAR. Having bony gills; and the ventral fins situated in 
front of the pectoral fins, as the cod, haddock, and whiting. 
III. THORACIC. Having bony gills ; and the ventral fins situated 
directly under the pectoral fins, as the perch and mackerel. 
IV. ABDOMINAL. Having bony gills; and the ventral fins on the 
lower part of the body below the pectoral fins, as the salmon, 
herrings, and carp. 
V. BRANCHIOSTEGOUS. Having their gills destitute of bony rays. 
VI. CHONDROPTERYGEOUS. Having cartilaginous fins, as the stur- 
geons, sharks, and skate. 
The fifth class of Linnasus comprised the Insects ; and the branch 
of zoology which treats of them is called Entomology. Nearly all 
insects go through certain great changes at different periods of their 
existence. From the egg is hatched the larva, which is a grub or cater- 
pillar, and destitute of wings ; this afterwards changes to a pupa, or 
chrysalis, wholly covered with a hard shell, or strong skin, from 
which the perfect or winged insect bursts forth. Spiders and their 
allies, which were included by Linnaeus in the insects, issue from 
the egg nearly in a perfect state. 
Linnaeus divided his insects into seven Orders. 
ORDERS OF INSECTS. 
I. COLEOPTEROUS. Having elytra, or crustaceous cases covering the 
wings; and which, when closed, form a longitudinal division 
along the middle of the back, as the cockchafer. 
II. HEMIPTEROUS. Having four wings, the upper ones partly crus- 
taceous, and partly membranous ; not divided straight down 
the middle of the back, but crossed, or incumbent on each 
other, as the cock-roacli. 
III. LEPIDOPTEROUS. Having four wings covered with fine scales, 
almost like powder, as the butterflies and moths 
