82 Proceedings of the Royal Society of Edinburgh. [Sess. 
is slightly warmer than the medium, in some cases it is of the same 
temperature, and in other cases it is even colder than the medium. 
I have never, on any occasion, found the body of the animal to be 
colder than the medium, except when the temperature of the water was 
slowly rising during the time that the records were being taken. This 
always happened when the examination was made in a pond of sea- water 
left behind by the receding tide ; the heat communicated by the hand 
to the limited quantity of water was sufficient to sensibly elevate its 
temperature several tenths of a degree in less than half an hour. The 
observations made on some crabs under these conditions were subsequently 
discarded as being inaccurate. In the deep and open water of the bay 
the temperature did not vary sensibly, and the animal was not once found 
to have a temperature below that of the water. 
The heat-loss from a fish surrounded by a medium of such high con- 
vective power as water must be very considerable and very rapid, more 
especially when it is not provided with any protecting covering, such as 
a layer of subcutaneous fat ; and there might be a large heat-production 
without any appreciable rise in temperature. As large fish would lose 
less heat, proportionately to body-weight, than small ones, having 
relatively a smaller radiating surface, it is in the former that one would 
expect to find a temperature difference, rather than in the latter, and such 
I find to be the case. 
The members of the tunny family of fishes show a remarkable peculiarity 
in this respect. John Davy found, in a specimen of the bonito ( Thynnus 
pelamis) captured near the equator, that the thermometer, when inserted 
into the deep muscles in the thickest part of the fish, recorded 37 0, 2 C., 
while the water from which it was taken was only 27 0, 2 C. This is a fish 
which rarely exceeds 3 feet in length. It is very active and swims 
near the surface, living chiefly on flying-fish. Its muscles are of a deep 
red colour, but I have been unable to find out from any description of 
this fish whether there exists an extensive deposit of subcutaneous fat or 
any other provision for the prevention of great heat-loss. The tunny 
of the Mediterranean ( Thynnus vulgaris) is a larger species, the adult 
measuring from 7 feet to 10 feet in length. It is also found in the warm 
regions of the Atlantic, and a few specimens have been captured in British 
waters. The Maltese fishermen recognise that this is a warm-blooded 
fish, and one of the most intelligent of them, when questioned as to the 
degree of heat, described it to Davy as “ much the same, or little less than 
that of the blood of a pig when flowing from the divided vessels of the 
neck on being killed.” 
