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. (i) 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 papillae, 

 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 Reptiles. 



(2) "Ganoid" scales, as in Lepidosteus, 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 

 som^ 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 Polypterus, 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, Polypterus^ and Amia, and where the gas 

 within is periodically emptied and 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 (i) 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 a chain of bones, as in Siluridae. This has suggested 



