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 



