INTRODUCTION^. 69 



found cmomonly in the mud at the bottom of the 

 water, and in the rays ; and among the osseous 

 fishes, it is wanting generally in the flat fishes, as 

 the turbot, and the rest already mentioned as inha- 

 bitants rather of the mud at the bottom of the 

 water, than of the water itself — in the launce or 

 sand-eel, already likewise alluded to, as inhabiting 

 the sand rather than the water — as well as in the 

 European angler (LopJdus jnscatoriiis ) ; which, 

 again, is one of the fishes described as in the habit 

 of burying itself in the soil, while lying in wait for 

 its prey. It appears to be wanting also in the 

 anchovy, the leaping blenny, the gudgeon, the 

 flying scorpasna, the sucking-fishes, the mackerel, 

 and many others. It will hence be observed, that 

 there is no precise correspondence between the 

 habits of fishes, as accustomed to remain at rest 

 at the bottom of the water, or to glide through 

 its bosom, and the absence or presence in them of 

 an air-bladder ; since, while, on the one hand, not 

 a feAV of them elsewhere spoken of, as found com- 

 monly in the mud or in the sand, are furnished 

 with this organ, many of those, on the other hand, 

 above specified as destitute of it, are still accustomed 

 to move freely through the water. 



The principal use of the air-bladder of fishes, 

 appears to be that of increasing or diminishing their 

 bulk, without changing their absolute weight, and 

 thus of modifying their specific gravity as circum- 

 stances may require ; and it is probably owing to 

 the circumstance of fresh water being lighter than 



