FINS. IX 



during some period of their existence may be found to have the outer 

 end articulated. Long and" delicate filaments are sometimes developed in 

 connection with fin-rays^ and often indicate that the example is young, 

 as in a horse mackerel, Garanx gallus, wherein their length diminishes 

 with age, similarly in the. dory; still they are occasionally continued 

 through life, as in an Indian barbel, Barhus filamentosus, and have likewise 

 been found existing in fishes taken from the deep and nearly still abysses, 

 of the ocean. Acanthopterygians are forms which, as a rule, possess 

 in their fins some true spines, which are absent in the soft-rayed or 

 malacopterygians. 



The fins are divisible into such as are single and unpaired, hence termed 

 "^azygous," as those along the median line of the back or dorsal, which 

 when present .may be variously subdivided or modified ; the tail or caudal fin 

 as a rule is placed vertically at the posterior extremity of the caudal portion 

 of the body, where it is seen in two very distinct types, in the 'generality; of 

 fishes the two lobes being, equal, when it is termed " homocei-cal," as in the 

 perch; whereas in the sharks and some other allied classes the vertebral 

 column is prolonged into the . upper half of the caudal fin, rendering it 

 unequally lobed or " heterocercal." The anal commonly commences behind 

 the vent, and passes, along the median line of the lower surface; it may be 

 as variously modified as the dorsal. These median or unpaired fins are. 

 considered by some to be appendages to the skin, but believed by Balfour 

 to be the specialized and highly-developed remnants of a once continuous 

 lateral fin along either side: 



But most fishes likewise possess two pairs of horizontal or paired fins, one 

 on the breast or pectorals corresponding with the anterior extremities of 

 higher vertebrata ; while the second or ventral pair, the homologues of the 

 legs or hind pairs of limbs, when present are variously placed, as under the 

 throat when they are termed jugular, as in the cod (plate IxxT^ii), below 

 the pectoral fins or thoracic, as in the perch (plate i), or abdominal when 

 near the hind end of the body, as in the carp (plate cxxix). 



Fins may likewise alter with age, thus their proportionate lengths and 

 development m the -young of abyssal or deep-sea fishes, as well as in pelagic 

 forms or those whose sphere of existence is usually restricted to the upper 

 waters of the open sea, would seem to be frequently very different to what 

 obtains in the adult, having caused the immature to be classed in a separate 

 genus from the same fish when it has attained to its full size. Thus the young . 

 of the abyssal Luvarus Guvieri (plate xliii and vol. i, page 120), has been 

 described as Diana semitunata. The pelagic sword-fishes (plate xlix and vol. i, 

 page 148) have in their earlier stages single, elongated, many-rayed dorsal and 

 anal fins, but the anterior portions of both become atrophied with age, while 

 the central rays disappear, thus leaving two fins in either position. In the pilot 



