Sir Everard Home on the structure of the organs , &c. 2 57 
The materials of the present Paper include five several 
links in the chain distinct from the gills of fishes, and diffe- 
rent from the organs common to vermes ; these are met with 
in the lamprey and lampern, in a new genus intermediate be- 
tween the lamprey and myxine, in the myxine, in the aphro- 
dita aculeata, and in the leech. 
I shall first give a short description of each of these organs, 
and afterwards explain the modes of respiration. 
In the lamprey the organs of respiration have seven exter- 
nal openings on each side of the animal ; these lead into the 
same number of separate oval bags placed horizontally, the 
inner membrane of which is constructed like that of the gills 
in fishes. There is an equal number of internal openings lead- 
ing into a tube, the lower end of which is closed, and the 
upper terminates by a fringed edge in the oesophagus. These 
bags are contained in separate cavities, and inclosed in a 
thorax resembling that of land animals, only composed of 
cartilages instead of ribs, and the pericardium, which is also 
cartilaginous, is fitted to its lower extremity like a diaphragm. 
In the middle line of the anterior part of the thorax are 
situated the muscles of the tongue, forming one solid mass, 
from which a distinct muscle is continued down to the peri- 
cardium, sending off fasciculi to the cartilages in the lower 
part of the thorax. 
There is but one nostril, which opens into a cavity of con- 
siderable size, having no posterior opening. Where the 
oesophagus terminates in the stomach, it adheres to the peri- 
cardium, and forms an oblique valvular slit, which is closed 
by the dilatation of the stomach. There is no gall bladder. 
In the lampern, the structure of these organs is the same 
mdcccxv. L 1 
