512 PISCES— FISHES. 



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 visceral arches. 



Two 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 archiptery- 

 gium ; and (6) the commoner type, in which the radials arise from a 

 number of basal pieces (an ichthyopterygium). Experts do not seem to 

 have yet come to a decision as to which of these types is the more 

 ancient, or as to how they are related to one another. 



Professor Huxley suggested that the fingered limb (cheiropterygium} 

 of higher Vertebrates might arise from a limb of the Ceratodus type by 

 an atrophy of its proximal fore-and-aft radials, and the hypertrophy of 

 its distal radials. Thus the axis becomes the middle digit, while the. 

 other four digits are the terminations of the two distal radials on each 

 side. But it seems just as easy or as difficult to trace the digitate limb 

 to an ichthyopterygium. 



Another interesting subject of inquiry is as to the origin of the girdles, 

 whether as ingrowths from the bases of the limbs, or from modifications 

 of branchial arches, or from both or neither. 



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 doisally 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. — 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 



