2 6o Sir Everard Home on the structure of the 
this, however, is unfounded, as the nostril has no communi- 
cation with the mouth. The elasticity of the cartilages of 
the thorax, admits the water being received, and it is expelled 
by the action of the muscles drawing up the cartilages and 
the pericardium. The animal from the South Seas having 
no cartilaginous thorax, the bags themselves have an elastic 
covering which keeps them open to receive the water, and it 
is expelled by the action of the external muscles into the 
oesophagus. 
In the myxine, the elasticity of the two tubes and the 
bags into which they open, admits of the water being received, 
and the pressure produced by the action of the external mus- 
cles forces it into the oesophagus, from whence it is thrown 
out by the opening at the lower end of that tube. 
Bloch has given a correct account of many parts of the 
myxine, illustrated by engravings, but there are several errors 
respecting the mode in which the water passes out. He 
supposes it to be thrown out at the nostril. He was probably 
led into this mistake from finding a posterior nostril commu- 
nicating with the mouth. 
In the aphrodita aculeata, the water passes through the 
lateral openings between the feet into the cavity under the 
muscles of the back ; it is there applied to the surfaces of 
the projecting cells, through which the air in the water is 
communicated to the caeca contained in them : these caeca I 
consider to be the respiratory organs. 
In the leech, the water is received through the openings 
on the belly of the animal into the cells, or respiratory organs, 
and passes out by the same openings. 
A knowledge of the mechanism employed for the pur- 
