142 HOUSE, GARDEN, AND FIELD 



of being full of bones. I know of no fish to match it, 

 though many fishes have both upper and lower ribs. These 

 outgrowths give excellent support to the muscles of the 

 trunk, but why they should be so very numerous is more 

 than I can explain. 



Just beneath the back-bone, and running almost from 

 head to tail, is the silvery air-bladder. It is possible by 

 careful search to find the air- duct which leads from the 

 stomach to the bladder, pass a pipe into it, and blow out 

 the bladder. The chief use of this air-receptacle is no 

 doubt to make the fish just so buoyant that it can keep 

 at the surface of the water without effort. Then it can 

 hold itself upright by the slightest exercise of its paired 

 fins, and propel itself by the sculling action of the tail. 

 It is worth while to study the action in a live gold-fish. 

 Many fishes have no duct to the air-bladder, and can only 

 fill it with gases drawn from the blood, but the herring's 

 air-bladder has not only a duct communicating with the 

 stomach, but a fine passage passing backwards and opening 

 on the surface of the body, as well as two slender tubes 

 which enter the head, and lead up to the organ of hearing. 

 The mode of action of these complicated passages is hardly 

 at all understood. It is of interest to remark that the 

 air-bladder of a fish is the beginning of the lung, by which 

 quadrupeds, birds, and reptiles breathe. We have all the 

 stages of development. First the bladder becomes cellu- 

 lar, that is, divided into compartments, then it becomes 

 double, then it acquires special blood-vessels leading to 

 and from the heart, and lastly, the simple air-duct becomes 

 elaborated into a wind-pipe, with rings of cartilage, and 

 perhaps an organ of voice. When you next see a boiled 

 cod or haddock at table, look for the air-bladder. It will 

 not of course be inflated with air, because all the air will 

 have been driven out by the heat, but you will recognise 

 it by its silvery coat. There is no air-duct in the cod or 

 haddock. 



A hard-roed herring is a female, and the hard roe con- 



