6 Sir Everard Home’s account of the 
at the distance of many hours, when exposed to heat, leap to 
a considerable distance; but, when once the circulation has 
ceased, there is no authentic account of the circulation being 
restored. The lampreys have a less degree of aeration of 
the blood than fishes, and in that respect become an inter- 
mediate link between them and vermes ; they have less the 
habits of muscular exertion, which may explain their having 
a less degree of aeration of the blood. 
The vermes of Linnaeus is a class made up of materials, 
which, in the present view of the subject, must be divided 
into five distinct orders. Those animals in which there is 
a heart; those in which there is no heart, but external organs 
of aeration ; those in which the circulation is carried on by 
the arteries and veins of the body, there being neither heart 
nor external organs of aeration ; those in which the blood 
does not circulate, but in which an undulation is kept up, a 
circulation for the purpose of aerating the blood being ren- 
dered unnecessary, as the aerating organs consist of air tubes 
that ramify through every part of the body, and those in 
which neither circulation nor undulation can be demonstrated. 
In all the classes of animals above the Vermes, the heart is 
employed to receive unaerated blood, and to propel that blood 
into the organs of aeration ; and in fishes, this is the only 
office it performs; but, in the class Vermes, the circulation 
is completely reversed, as I have formerly explained in the 
teredines, since the aerated blood goes to the heart, which 
propels it to the different parts of the body. 
In so small an animal as the Teredo navalis, all the peculiar- 
ities of this kind of circulation were not readily made out, but 
by examining it in the sepia officinalis of a large size, I find 
that there is the same change in the office of the blood vessels 
