Dorsal and anal fins 



In actinopterygian fishes there is a great variety of fin 

 structures and fin arrangements. The dorsal fin may be 

 preceded by a series of spines, as in the perch; this fin may 

 be divided into two parts or it may be continuous with the 

 caudal fin. The salmon has a small "adipose" fin between 

 dorsal and caudal. This small fin seems to be a new struc- 

 ture peculiar to some of the teleosts. The skeletal support of 

 the dorsal fin or its parts is fairly simple. Radials extend up- 

 ward from the spinal column toward the fin, and on the 

 ends of these there are one or two basal elements which 

 support the fin rays or spines (Figure 6-40). The anal fin is 

 similarly constructed. 



In the more primitive (fossil) actinopterygians, there was 

 a single dorsal fin, an arrangement retained in Acipenser 

 (Figure 2-3) or Lepisosteus (Figure 2-4). This style is obscured 

 in Amia (Figure 2-4) where the dorsal extends much of the 

 length of the body, and in Polyptenis (Figure 2-5) where 

 many finlets replace the usual dorsal fin. These finlets oc- 

 cupy much the same space as the dorsal fin oi Amia, but 

 there is no gap between them and the caudal fin. Most acti- 

 nopterygians have a simple anal fin, but sometimes it be- 



comes continuous with the caudal or is subdivided into fin- 

 lets. 



In the choanate fishes (Figure 2-18), there were two dor- 

 sals, the more posterior one generally the larger. There was 

 a single anal in the same position as that of the actinoptery- 

 gian. Latimena (Figure 2-6) has brought this pattern down 

 to the present, differing in that the anterior dorsal is the 

 larger. These fins in the rhipidistian or actinistian have 

 large, plate-like basal structures, with or without radials, in 

 lobe-like extensions of the body resembling those of the 

 lateral fins (Figure 6-41). The dipnoans retain the simplest 

 relationship between basal, radial, and fin ray. 



The Chondrichthyes (Figure 2-2) generally have two 

 dorsals but the most primitive living genera (Chlamydose- 

 tachus, Heptanchus, Hexanchus) have only one. This excep- 

 tional condition may be due to secondary loss of one fin, or 

 evidence of both patterns of dorsal fins in sharks. The 

 known Devonian species of sharks (Figure 2-15) had two 

 fins and this suggests that loss has occurred in these primi- 

 tive living genera. The Holocephalans have two dorsals; the 

 second is long and low in Hydrolagus. The skeletons of these 

 fins are complex in the shark, resembling those of the late- 

 ral fins or being simply a jumble of closely packed carti- 

 laginous elements. The anterior dorsal of Hydrolagus has a 



dorsal fin 



adipose fin 



eleventh vertebra neural spine 



Figure 6-40. Middle region of vertebral column of a lantern fish, Lompanycfus /eucopsorus, along 

 with skeletons of the dorsal and anal fins. 



MEDIAN FINS • 167 



