Pi 
INTRODUCTION. ol 
merely attached to the skin, which constitutes a soft contrac- 
tile envelope, in which, in many species, are formed stony 
plates, called shells, whose position and production are ana- 
logous to those of the mucous body. ‘The nervous system is 
contained within this general envelope along with the viscera, 
and is composed of several scattered masses connected by ner- 
vous filaments; the chief of these masses is placed on the 
cesophagus, and is called the brain. Of the four senses, the 
organs of two only are observable, those of taste and sight, the 
latter of which are even frequently wanting. One single 
family alone presents organs of hearing. ‘There is always, 
however, a complete system of circulation, and particular or- 
gans for respiration. ‘Those of digestion and secretion are 
nearly as complex as in the vertebrata. We will distinguish 
the animals of this second form by the appellation of 
Animalia Mollusea. 
Although, as respects the external configuration of the 
parts, the general plan of their organization is not as uniform 
as that of the vertebrata; there is always an equal degree of 
resemblance between them in the structure and the functions. 
The third form is that remarked in worms, insects, &c. 
Their nervous system consists of two long cords, running lon- 
gitudinally through the abdomen, dilated at intervals into 
knots or ganglions. The first of these knots, placed over the — 
cesophagus, and called brain, is scarcely any larger than those | 
that are along the abdomen, with which they communicate by 
filaments that encircle the cesophagus like a necklace. The 
covering or envelope of the body is divided by transverse 
_ folds into a certain number of rings, whose teguments : are ‘ 
sometimes soft, and sometimes hard; the muscles, however, 
‘being always situated internally. Articulated limbs are fr ia 
quently attached to the trunk ; but very often there ar 
We will call these animals 
‘ Animalia ee v4 hing Aidt 
a cay Hyatt 
Or articulated animals, in hem is observed the transition 
