THE BREATHING APPARATUS. 27 
waistcoat, if we suppose the lining of the flaps to be made 
in one piece with the sides of the waistcoat. Or a closer 
parallel still would be brought about, if the skin of a 
man’s back were loose enough to be pulled out, on each 
side, into two broad flaps covering the flanks. 
It will be observed that the branchial chamber is open 
behind, below, and in front; and, therefore, that the water 
in which the crayfish habitually lives has free ingress 
and egress. Thus the air dissolved in the water enables 
breathing to go on, just as it does in fishes. As is the 
case with many fishes, the crayfish breathes very well 
out of the water, if kept in a situation sufficiently cool 
and moist to prevent the gills from drying up; and 
thus there is no reason why, in cool and damp weather, 
the crayfish should not be able to live very well on land, 
at any rate among moist herbage, though whether 
our common crayfishes do make such terrestrial excur- 
sions is perhaps doubtful. We shall see, by-and-by, that 
there are some exotic crayfish which habitually live on 
land, and perish if they are long submerged in water. 
With respect to the internal structure of the crayfish, 
there are some points which cannot escape notice, how- 
ever rough the process of examination may be. 
Thus, when the carapace is removed in a crayfish 
which has been just killed, the heart is seen still 
pulsating. It is an organ of considerable relative size 
(fig. 5, h), which is situated immediately beneath the 
