GENERAL NOTES ON STRUCTURE OF FISHES. 513 



forming the rest. It has been recently maintained, however, that the 

 ectoderm forms most, if not all, of the scale (see p. 430). In other 

 fishes the scales are almost wholly dermic, in marked contrast to those 

 of Reptiles. 



In most Teleosteans the scales are soft, and the epidermic covering is 

 very thin. They are called cycloid or ctenoid, as their free margins 

 projecting from sacs in the dermis are entire or notched. But bony 

 scales also occur in many Teleosteans. 



The sturgeon has five rows of bony dermic plates (scutes) ; the scales 

 of the Bony Pike (Lepidosteus), Polypterus, and many extinct Ganoids, 

 are covered with enamel. 



The great interest of these exoskeletal structures is that those of 

 Elasmobranchs are homologous with teeth, and that many bony scales 

 often fuse into plates, suggesting the manner in which the membrane 

 bones of the skull and pectoral girdle (e.g. the clavicle of Bony Fishes) 

 are believed to have originated. 



The simplest teeth of Elasmobranchs are precisely homologous with 

 dermal denticles. But just as the skin-teeth sometimes fuse in groups, 

 so is it also with their homologues, which form true teeth. Compound 

 cuspidate teeth in sharks arise from the fusion of adjacent simple cusps. 

 But the fusion may go further ; a complex crushing dental plate may be 

 formed from the coalescence of several successional teeth. A further 

 complication is brought about by the multiplication of cusps on the in- 

 dividual teeth. These facts are, as Mr. A. Smith Woodward points 

 out, of much interest, because it is by similar processes of fusion and of 

 multiplication that the complex teeth of various Mammals arise. 



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 foims 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, Lepidosirai, 

 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 — (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. (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 



.33 



