MEDIAN FINS 



235 



the expense of the middle and distal segments. The cause of 

 this reduction is often, but not always, to be found in the 

 fact that, whenever the dermal fin-rays take the form of stout 

 spines, as in the anterior dorsal fin in many Acanthopterygiau 

 Teleostei, the segmentation of their radialia would obviously 

 detract from their value as skeletal supports, and hence they 

 rarely consist of more than their proximal segments, although 

 the radialia which in the same Fish support soft rays may be 

 bi-segmented or tri-segrnented. The radialia are, however, uu- 

 segrnented, even slightly branched, cartilaginous rods in the 

 Cyclostomata ; short simple rods in the Holocephali ; and equally 

 simple bony rods in the dorsal fin of Polypterus, where they sup- 



FIG. 136. The tri-segmented radialia and the fin- 

 rays of part of the dorsal fin of Amfa calm, 

 p.s, m.s, and d.s, The proximal, middle, and 

 distal segments of a radial ; /.?, fin-rays. (From 

 Bridge. ) 



FIG. 137. The first four radialia 



of the dorsal fin of Mesoprion 

 yembra, showing the chain- 

 links for the ring-like bases of 

 the fin-rays. r.e\ r.e\ First 

 and fourth proximal radialia. 



port the strong spines of the numerous finlets ; but they are bi- 

 segmented in the soft-rayed anal fin. As previously mentioned, 

 the proportional share taken by the radialia and the horny fibres 

 or the dermal fin -rays in the support of the fins differs greatly 

 in different Fishes. In the Cyclostomata radialia are the sole, 

 and in Elasmobranchs the main supports, and they may extend 

 nearly to the free margin of the fin. In the more specialised 

 Fishes, as in most Teleostomi, the reverse is the case. The 

 radialia sink into the muscles of the body-wall and leave the 

 strongly developed fin-rays as the sole support of the visible 

 portions of the fins. In not a few Fishes there is an obvious 

 segmental correspondence between the radialia and the vertebral 

 neural or haemal spines, to the extent that the former equal the 

 latter in number and articulate with their distal extremities, as, 

 for example, in the caudal region of Pleuracanthus and in existing 



