GENERAL NOTES ON STRUCTURE OF FISHES. 565 
The effect of a stroke with the heterocercal tail is to force the anterior 
region downwards, and thus the heterocercal tail in fish is associated 
with a ventral mouth and the habit of ground-feeding. The movement 
of the homocercal tail, on the other hand, drives the body straight 
forwards, and is associated with a terminal mouth. _ 
Scales.—(1) In Elasmobranchs the scales (placoid) have the form of 
skin-teeth (dermal denticles), tipped with enamel, cored with dentine, 
and based with bone sunk in the dermis, They arise from skin papillee, 
the (ectodermic) epidermis forming the enamel, the (mesodermic) dermis 
forming the rest. In other fishes the scales are almost wholly dermic, 
in marked contrast to those of Reptiies. 
(2) ‘*Ganoid” scales, as in Lepzdosteus, are plates of bone with an 
enamel-like covering called ganoin. . 
(3) In most Teleosts the scales are relatively soft dermic plates 
of thin bone. In the sturgeon and many Teleosts the scales are 
substantial bony plates. The typical ‘‘soft” Teleost scales are called 
cycloid or ctenoid, as their free margins projecting from sacs in the 
dermis are entire or notched. The concentric rings on the scales 
indicate periods of growth, like the rings on a tree stem, and it is 
possible in some cases to tell the age of a fish from its scales, as also 
from the otoliths in the ear when these have a layered structure. 
_ The scales “of Elasmobranchs are homologous with teeth, and a 
number may fuse into'a plate just as teeth often do. 
~ Swim-bladder.—The swim-bladder of fishes is one of the 
numerous outgrowths of: the gut. It is absent in Elasmobranchs and 
some Teleosteans, such as most flat-fish, and it forms the lung of 
Dipnoi. Unlike a lung, it opens dorsally into the gut, except in 
Dipnoi and the Ganoid olypterus, where the aperture is ventral. 
The original duct communicating with the gut may remain open, as in 
Physostomatous Teleosteans, or it may be closed, as in Physoclystous 
Teleosteans. The bladder is usually single, but it is double in 
Protopterus, Lepidosiren, and Polypterus. 
In regard to the use of the swim-bladder, there is still considerable 
uncertainty. Where it is abundantly supplied with impure or partially 
purified blood, as in Dipnoi, Polygterus, and Ama, and where the gas 
within is periodically emptied arid renewed, it is doubtless respiratory. 
But what of other cases, where its supply of blood is arterial, and what 
especially where it is entirely closed? In such cases it is usual to speak 
of its function as hydrostatic. 
In greater detail the function of the air-bladder is—(1) to render the 
fish, bulk for bulk, of the same weight as the medium in which it lives ; 
moreover (2), the volume of the contained gas varies with increased 
secretion and absorption, and seems to adjust itself to different external 
pressures as the fish descends or ascends, There is sometimes a well- 
developed gas-gland with a rich blood-supply on the inner wall of the 
bladder. (3) In many fishes the bladder may help indirectly in 
respiration by storing the superabundance of oxygen introduced into 
the blood by the gills. (4) There is in several Teleosteans a remarkable 
connection between the swim-bladder and the ear, sometimes by an 
anterior process of the bladder, as in the herring and perch-like fishes, 
sometimes by 4 chain of bones, as in Siluride. This has suggested 
