VERTEBRATA : FISHES. 469 



bones of the scapular arch, and is therefore wholly removed 

 from its proper vertebra. 



In addition to the pectoral and ventral fins the homologues 

 of the limbs which may be wanting, fishes are furnished with 

 certain other expansions of the integument, which are "me- 

 dian" in position, and must on no account be confounded 

 with the true " paired " fins. These median fins are variable 

 in number, and in some cases there is but a single fringe run- 

 ning round the posterior extremity of the body. In all cases, 

 however, the median fins are " azygous " that is to say, they 

 occupy the middle line of the body, and are not symmetrically 

 disposed in pairs. Most commonly, the median fins consist 

 of one, two, or three expansions of the dorsal integument, 

 called the " dorsal fins" (fig. 252, d, d')-, one or two on the 

 ventral surface near the anus the "anal fins" (fig. 252, a)-, 

 and a broad fin at the extremity of the vertebral column, called 

 the " caudal fin " or tail (c). In all cases, the rays which support 

 the median fins are articulated with the so-called interspinous 

 bones, which have been previously described. Though called 

 " median," from their position in the middle line of the body, 

 and from their being unpaired, the median fins of fishes, as 

 shown by Goodsir and Humphry, are truly to be regarded as 

 formed by the coalescence of two lateral elements in the mesial 

 plane of the body. 



The caudal fin, or tail, of fishes is always set vertically at the 

 extremity of the spine, so as to work from side to side, and it 

 is the chief organ of progression in the fishes. In its vertical 

 position, and in the possession of fin-rays, it differs altogether 

 from the horizontal integumentary expansion which constitutes 

 the tail of the Whales, Dolphin, and Sirenia (Dugong and 

 Manatee). In the form of the tail, fishes exhibit some striking 

 differences. In some of the Bony Fishes and Ganoids, the 

 caudal extremity of the spine is not bent upwards, but divides 

 the caudal fin-rays into two nearly equal portions, and the 

 symmetrical tail-fin thus produced is said to be " diphycercal." 

 In the great majority of the Bony Fishes the tail-fin appears 

 on inspection to be divided into two equal lobes, and it is 

 then said to be "homocercal" (fig. 253, A). This apparent 

 symmetry is due to the fact that the spinal column seems to 

 terminate in the centre of a triangular bony mass, to the free 

 edges of which the fin -rays are symmetrically attached. In 

 reality, however, the unossified notochord is prolonged into the 

 upper lobe of the tail ; and as there is a much larger number 

 of fin-rays below the bent-up notochord than above it, the tail 

 is truly unsymmetrical in its fundamental structure. Lastly, in 



