organs of respiration in animals. 259 
ings on each side in the intermediate spaces between the tufts of 
bristles ; these all open into a large cavity immediately under 
the skin and muscles of the back, which is only separated 
from the cavity of the abdomen by a strong cartilaginous 
membrane, but there are two rows of spherical cells, fifteen 
in each, projecting into the cavity with very thin membranous 
coats. There is no external opening into them, but a slit on 
the under surface, by means of which, one of the caeca be- 
longing to each of the tubes passing off from the intestine is 
lodged in each of the cells, which leads me to consider such 
caeca to be the respiratory organs. 
In the common leech, there are sixteen orifices on each 
side of the belly, which lead to an equal number of spherical 
cells placed between the abdominal muscles and the stomach, 
which perform the office of respiratory organs. The parti- 
cular structures which have been described, are represented 
in the annexed drawings, (Plates XI. XII. XIII. ) which makes 
a more detailed verbal description unnecessary. 
Having described the structure of the organs in these five 
different genera of animals, I shall endeavour to explain the 
manner in which respiration is carried on in each. 
In the lamprey and lampern, the water is received by the 
seven lateral openings on each side of the animal into the 
bags which perform the office of gills, and passes out by the 
same orifices. The form of the cavities being fitted to allow 
the water to go in at one side, pass round the projecting parts, 
and out at the other. A part of the water escapes into the 
middle tube, and from thence, either passes into the other 
bags, or out at the upper end into the oesophagus. There is a 
common opinion that the water is thrown out of the nostril ; 
LI 2 
