INTRODUCTION. XX111 
naean genera Julus and Scolopendra, commonly called centi- 
pedes. 
The term larva is applied to the young of all insects included in 
the first seven orders, when first hatched. The different kinds have, 
however, other names ; that is to say, the larva of a butterfly, or 
moth, is called a caterpillar ; that of a beetle, a grub ; and that of 
a fly, a maggot. The larva changes its skin several times, and at 
last it goes into the pupa state, when it is called a chrysalis, an 
aurelia, or a nymph. Sometimes the pupa is wrapped up in a loose 
outer covering called a cocoon. From the pupa in time bursts forth 
the imago, or perfect insect. The Apterous, or wingless true in- 
sects, and the Myriapoda, which are also without wings, do not un- 
dergo any metamorphosis. Entomology is a very difficult study, 
from the great number of systems that have been broached by mo- 
dern naturalists, and from the changes continually making in the 
names of the insects. 
THE RADIATED ANIMALS 
Are so called because their organs of locomotion, and even their in- 
ternal viscera are generally arranged in a circle round a centre, so 
as to give a radiated appearance to the whole body, The animals 
included in this class are the very lowest in the scale; they have 
scarcely any external senses; their movements are slow, and almost 
their only sign of life is a craving for food. Some of them, however, 
have a distinct mouth and alimentary canal, with an anal orifice ; 
others have a bag-like stomach with a kind of mouth, through which 
they both take their food and reject their excrements ; while others 
have no mouth, and only appear to absorb nourishment through 
pores. In the like manner, though some are oviporous, others may 
be propagated by division like plants. There are five classes, 
which are as follows. 
I. Echinodermata, or sea-urchins. These animals have a leathery 
skin, or crustaceous skin or shell, commonly covered with 
numerous tubercles. The mouth is in the centre of the ani- 
mal, and is armed with five or more pieces of bone, which 
serve as teeth ; the stomach is a loose bag ; the organs for res- 
piration are vascular ; and the animals are oviparous. They 
are furnished with tentacular tubes, which serve as arms or 
feet, and which they can push out and draw back at pleasure ; 
and they have yellowish or orange-coloured blood, which 
appears to circulate. Cuvier divides this class into those 
with feet, and those without ; but Lamarck, whose arrangement 
