ON THE AIR-BLADDERS OF FISH. 97 
as on the South Pacific Coast, from Peru to the Straits of 
Magellan, there is another species (Stercorarius chilensis), which 
is more closely allied to S. catarrhactes than to S. antarcticus, a 
third species of Great Skua, which also inhabits the southern 
hemisphere, ranging from New Zealand, throughout the Antarctic 
Regions and by way of Kerguelen, the Crozets, &c., to the 
Falkland Islands and the east coast of Patagonia. On its 
migration the northern Great Skua occurs on both sides of the 
Atlantic, but it has not been traced farther south than the 
Straits of Gibraltar, and information is much desired. It is 
rather an easy species to recognise even on the wing, both from 
its habit of pursuing and plundering the Gulls and Gannets, as 
well as from its bold sweeping flight, in which the white mirror 
on the primaries is a very marked feature; so that some of our 
naval officers who take an interest in the birds they come across 
at sea may perhaps be able to supply details respecting it. 
In this sketch of the Skuas I have endeavoured, whilst 
avoiding technical descriptions and details, to give those par- 
ticulars which appeared most likely to awaken interest and to 
stimulate research. How much still remains to be learned upon 
the geographical dtstribution of this sub-family will be only too 
manifest to all who have read these remarks. 
ON THE AIR-BLADDERS OF FISH. * 
By Francis Day, F.L.S., F.Z.S. 
Amone the organs existing in fish, few have given rise to 
more speculation and discussion than the air- or swim-bladder, 
also termed air-sac or air-yessel, which is a single or variously 
subdivided sac, or it may be two sacs partially or completely 
separated one from the other. Situated above the centre of 
gravity, it is beneath the vertebral column or back-bone, from 
which it is generally more or less divided by the kidneys: while 
inferiorly it is separated from the intestines by the peritoneum. 
As this air-bladder may be present or absent in species belonging 
to the same genus, and is frequently wanting, it is evident that it 
cannot be indispensable to a fish’s existence, that its functions 
* Summary of a Paper read before the Cotteswold Naturalists’ Field Club, 
17th February, 1880. 
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