GREA T BRITAIN. 17 



Taking a general survey of fishes, we find that caecal 

 appendages are far more common among marine than 

 fresh-water fishes, and that in the sharks and rays instead 

 of these appendages a spiral valve, already described, exists 

 in their intestines. 



We find members of the cod family, which are more or 

 less ground-feeders, have large numbers of caecal appen- 

 .dages ; so have also the surface-swimming herrings, while 

 the slow-moving flat-fishes (Pleuronectidcz) possess very 

 few. But the sluggish dory (Zeus) and the slow-swimming 

 lumpsucker (Cyclopterus) have many, and such variations 

 in forms which have somewhat similar habits is by no 

 means uncommon. Thus in some species of sea-perches 

 (Serrani) living in the same localities, the number of these 

 appendages may vary from eight to fifty. That a rich or 

 augmented diet may increase the number of these appen- 

 dages has been proved by sending out trout to Tasmania, 

 where they have, due to good living, grown to a size not 

 attained in this country by their ancestors, while the 

 number of caecal appendages has increased, showing that 

 these appendages are inconstant as to their numbers. 



The air- or swim-bladder, likewise termed the air-sac or 

 air-vessel, is a single or variously sub-divided sac, or it may 

 be two sacs partially or completely separated one from the 

 other. Situated above the centre of gravity, it lies beneath 

 the vertebral column or backbone, from which it is more or 

 less divided by the kidneys, while inferiorly the peritoneum 

 is between it and the intestines. As this organ is entirely 

 absent, or ceases to be developed, in many fishes, and may 

 be present or wanting in species belonging to the same 

 genus, it would appear that it is not indispensable to the 

 existence of these animals, its functions being accessory or 

 . VOL. I. E. I. C 



