24 INTRODUCTION. 
family alone presents organs of hearing. There is always, however, a 
complete system of circulation, and particular organs for respiration. 
Those of digestion and secretion are nearly as complex as in the verte- 
brata. We will distinguish the animals of this second form by the ap- 
pellation of 
Animalia Mollusca. 
Although, as respects the external configuration of the parts, the gene- 
ral 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 struc- 
ture and the functions. 
The third form is that remarked in worms, insects, &c. Their nervous 
system consists of two long cords, running longitudinally through the ab- 
domen, 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 esophagus 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 
frequently attached to the trunk; but very often there are none. We will 
call these animals 
Animalia Articulata, 
Or, articulated animals, in which is observed the transition from the cir- 
culation in closed vessels to nutrition by imbibition, and the corresponding 
one of respiration in circumscribed organs, to that effected by trachee or 
air vessels distributed throughout the body. In them, the organs of taste 
and sight are the most distinct; one single family alone presenting that of 
hearing. Their jaws, when they have any, are always lateral. 
The fourth form, which embraces all those animals known by the name 
of zoophytes, may also properly be denominated 
Animalia Radiata, 
Or, radiated animals. We have seen that the organs of sense and mo- 
tion in all the preceding ones are symmetrically arranged on the two sides 
of an axis. There is a posterior and anterior dissimilar face. In this 
last division, they are disposed like rays round a centre; and this is the 
case even when they consist of but two series, for then the two faces are 
similar. ‘They approximate to the homogeneity of plants, having no very 
distinct nervous system or particular organs of sense; in some of them, it 
is even difficult to discover a vestige of circulation; their respiratory or- 
gans are almost universally seated on the surface of the body, the intestine 
in the greater number is a mere sac without issue, and the lowest of the 
