INTRODUCTION. Xxi 
shell, and fringed appendages in the tail to retain the eggs. 
This order contains the crab family, which have an orbicular 
body and imperfectly developed tail ; and the lobster family, 
including the shrimp and the prawn, which have a cylindrical 
body, and a fully-developed tail. 
2. The Heterobranchia, which consist chiefly of the wood-lice, the 
brine-worm, and the ship-worm. The curious fossil animals, 
called trilobites, belong to this division. 
III. The Arachnides are defined by Lamarck to be oviparous 
animals, provided with six or more articulated legs, not sub- 
ject to metamorphoses, and never acquiring any new kinds 
of organs. Their respiraton is either by means of air- 
sacks, which serve for lungs, or of a kind of tube with circu- 
lar openings for the admission of air. There is a rudimen- 
tary heart and circulation in most of the species. There are 
two orders ; those with lungs, and those without. 
ORDER I. Pulmanarice. The Arachnides comprised in this divi- 
sion have air-sacks, which serve for lungs, a heart with dis- 
tinct vessels, and from six to eight simple eyes. Tnere are 
two distinct families : viz., Aranides, comprising all the spiders 
and spinners ; and Pedipalpi, comprising the tareutula and 
scorpions. 
ORDER II. Trachearice. These Arachnides are distinguished by 
their respiratory organs, which consist of radiated or 
branched tracheae, receiving air by two circular openings. 
They have no circulation, and their eyes vary from two to 
four. The principal animals belonging to this division are 
the long-legged spiders (Phalangium), the book-worms, and 
the mites (Acarus), including the gardener's pest the little 
red spider, (Acarus telarius), the cheese mite (Acants Siro) 
and the harvest bug, (Acarus or Leptus autumnalis.) 
IV. The Insects form the fourth and last class of articulated ani- 
mals, and they derive their name from the Latin word 
insectum, which signifies cut into, in allusion to the distinct 
divisions of head, thorax, and abdomen in the true insects ; 
and in contradistinction to the Annelides, the bodies of 
which present no such divisions. The true insects are de- 
fined as animals without vertebrae, possessing six feet, with 
a distinct head furnished with antennae, and breathing 
through stigmatic openings, which lead to interior tracheae. 
The Myriopoda have, however, more feet. The following 
are the twelve orders into which this class is divided. 
SECTION I. -Insects undergoing Metamorphoses. 
1. Coleoptera, (from two Greek words signifying sheathed wings.) 
These are the beetles, which are all furnished with mem- 
braneous wings, with which they fly, and which are protected 
by bony upper wings, or wing-cases, called elytra. They are 
all masticators, and are all provided with mandibles or pro- 
jecting jaws, and maxillae. 
