520 FISHES. 



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 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 Teleos- 

 teans, 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 communi- 

 cating with the gut may remain open, as in Physostomatous Teleosteans, 

 or it may be closed as in Physoclystous Teleosteans. The bladder is usu- 

 ally 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. (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 the view that the connection serves to 

 make the fish aware of the varying tensions of gas in the bladder, due to 

 the varying hydrostatic pressure, and in the same connection it is 

 interesting to notice the theory that the ear of fishes has to do through 

 its semicircular canals with the equilibration and orientation of the 

 animal's movements. It is also worthy of note that those fresh water 

 fishes (Ostariophysise), which have the adjusting mechanism above 

 referred to, have a marked ascendancy over all other fresh water species 

 in which this mechanism is awanting (Bridge and Haddon). 



Flat fishes. 



In illustration of biological problems, let us briefly discuss 

 some of the peculiarities of the flat fishes (Pleuronectidae), 



