GENERAL NOTES ON STRUCTURE OF FISHES. 543 



unpaired fins in their general structure, and in their mode of origin. 

 In some Elasmobranch embryos, Balfour showed that the pectoral and 

 pelvic fins were connected by transitory lateral ridges. It is therefore 

 possible that the paired fins may have arisen by a localisation of two 

 once continuous lateral folds. According to another theory, the origin 

 of paired fins is to be found in the visceral arches. 



The paired fins are supported by horny or bony fin-rays (dermotrichia] 

 and by endoskeletal pieces (somactids or radials), some of which are 

 articulated to the girdles and are then called basalia. Two main types 

 of fish fin are distinguishable (a) that best illustrated among living 

 fishes by Ceratodus, in which a median jointed axis bears on each side 

 a series of radial rays a form often called an archipterygium ; and (b] 

 the commoner type, in which the radials arise on one side of the basal 

 pieces (an ichthyopterygium). In the bony fishes the support of the 

 fin beyond the base seems mainly due to dermal rays. 



Tail. In Dipnoi and a few Teleosteans, e.g. the eels, the vertebral 

 column runs straight to the tip of the tail, dividing it into two equal 

 parts. This perfectly symmetrical condition is called diphycercal or 

 protocercal, but it is not quite certain that its thorough symmetry is 

 primitive. 



In Elasmobranchs, Holocephali, cartilaginous and many extinct 

 " Ganoids," the vertebral column is bent dorsally at the end of the tail, 

 and the ventral part of the caudal fin is smaller than, and at some little 

 distance from, the upper part. This asymmetrical condition is called 

 heterocercal. 



In most Teleostei, and in extant bony " Ganoids," the end of the verte- 

 bral column is also bent upwards, but the apex atrophies, and, by the 

 disproportionate development of rays on the ventral side, an apparent 

 symmetry is produced. The vertebral column usually ends in a urostyle, 

 the undivided ossified sheath of the notochord. Most of the fin 

 really lies to the ventral side of this. The condition is termed 

 homocercal. 



As to the mechanical importance of the different forms of the tail, 

 there are some interesting recent observations. 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 papilla:, 

 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 Lepidosteits, are plates of bone with an 

 enamel-like covering called ganoin ; they are wholly dermic. 



(3) In most Teleosts the scales are relatively soft dermic plates 

 of thin bone without enamel or ganoin. 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 pro- 



