INTRODUCTION. 33 
which constitutes a soft, contractile envelope, in which, in many species, are formed 
stony plates, called shells, the production and position of which are analogous to that 
of the mucous body ; the nervous system is contained within this general envelope, 
together with the viscera, and is composed of several scattered masses, connected by 
nervous filaments, and of which the principal, placed over the oesophagus, bears the 
name of brain. Of the four senses, the organs of those of taste and vision only can be 
distinguished ; the latter of which are even frequently wanting. A single family 
alone presents organs of hearing. There is always, however, a complete system of 
circulation, and particular organs for respiration. Those of digestion and of the secre- 
tions are little less complicated than in the vertebrated animals. We will distinguish 
the animals of this second form by the appellation of 
Molluscous Animals {Animalia mollusca). 
Although the general plan of their organization is not so uniform, as regards the 
external configuration of the parts, as that of the vertebrates, there is always an equal 
degree of resemblance between them in the essential structure and the functions. 
The third form is that observed in insects, worms, &c. Their nervous system con- 
sists of two long chords running longitudinally through the abdomen, dilated at inter- 
vals into knots or ganglions. The first of these knots, placed over the oesophagus, 
and called brain, is scarcely any larger than those which are along the abdomen, with 
which it communicates by filaments that encircle the msophagus like a collar. The 
envelope of their trunk is divided by transverse folds into a certain number of rings, of 
which the teguments are sometimes hard, sometimes soft, but to the interior of which 
the muscles are always attached. The trunk often bears on its sides articulated 
limbs, but is frequently unfurnished with them. We will bestow on these animals 
the term 
Articulate Animals {Animalia articulata). 
It is among these that the passage is observed from the circulation in closed vessels, 
to nutrition by imbibition, and the corresponding transition from respiration in cir- 
cumscribed organs, to that effected by tracheae or air-vessels distributed through the 
body. The organs of taste and vision are the most distinct in them, a single family 
alone presenting that of hearing. Their jaws, when they have any, are always lateral. 
Lastly, the fourth form, which embraces all those animals known under the name of 
Zoophytes, may be designated 
Radiate Animals {Animalia radiata). 
In all the preceding, the organs of sense and motion are arranged symmetrically on 
the two sides of an axis. There is a posterior and an anterior dissimilar face. In this 
last division, they are disposed as rays round a centre ; and this is the case, even when 
they consist of but two series, for then the two faces are alike.* They approximate to 
the homogeneity of plants, having no very distinct nervous system, nor organs of 
particular senses : there can scarcely be perceived, in some of them, the vestiges of a 
* M. Agassiz has expressed a different opinion. See Radiata. — Ex>. 
