X1V INTRODUCTION. 
over different parts of their bodies. The principal mass, 
which is situated transversely under the cesophagus, and 
round which it forms a nervous collar, is termed the brain. 
Their organs of motion and of sensation have not the 
same uniformity in number or position with those in the 
vertebrose animals: the variety is especially remarkable 
respecting the viscera, and more especially in the position 
of the heart and the respiratory organs: some of them 
respire the elastic atmospheric air, and others respire the 
air contained in fresh or salt water. 
The circulation of these animals is always double; that 
is to say, that their pulmonary circulation always makes a 
circuit aside and complete: this function is aided by a 
fleshy ventricle, not situated, as im the fishes, between the 
veins of the body and the arteries of the branchize (gills) ; 
but, on the contrary, between the pulmonary veins and the 
arteries of the body: this forms an aortic ventricle. In 
the class Cephalopoda, they are furnished with a pulmo- 
nary ventricle, which divides itself mto two. The aortic 
ventricles in the class Brachiopoda, and the family Arcade 
of the class Ginglymaconcha, divide as in the other genera ; 
but in all the other families of the class Ginglymaconcha, 
the auricle alone is divided. 
When they have more than one ventricle, they are not 
united together, as in the animals with warm blood, but 
frequently are widely separated from each other, and may 
then seem to have many hearts, to those who have exa- 
mined them cursorily. 
The blood of these animals is white or blueish, and the 
fibrine is less abundant than in that of the vertebrose 
animals. It is probable that their ves may sometimes 
perform the functions of absorbent vessels. 
Their muscles are attached to various parts of their skin, 
and form tissues less complicated, but more serrated, than 
in vertebrose animals. 
