174 



KNOWLEDGE. 



[September 1, 1892. 



Fig. 2. — Skeleton of tlie Percli. a — c, ^aws; rf, eye; e f, portions 

 g 1)', backbone ; h, pectoral fin ; i, pelvic fin ; k /, first anil second 

 w, anal fin ; » «', tail ; /' and )' are the paired fins. 



tail begins, and that the rays of the latter start from the 

 expanded end of the backbone itself. 



The same general type of structure obtains in the paired 

 fins of some of the later ganoids, sucli as the living 

 sturgeons, as well as certain extinct forms, and it is also 

 universally present in all the living sharks and rays. If, 

 however, we were to go to the Natural History Museum 

 and examine the baramunda of the rivers of Queensland, 

 or the gar-pike ( Lepidastewi) of those of North America, or 

 the bichir {I'ohjjiti'nis) of the Upper Nile and the rivers of 

 western equatorial Africa, we should tind a totally dif- 

 ferent structure obtaining in the paired fins. In all these 

 three fishes (of which, be it carefully noted, the first is a 

 representative of the lung-fishes, while the second and 

 third are ganoids) the first pair, or pectoral fins, as is well 



shown in the bichir repre- 

 sented in Fig. 1, are seen 

 to have a long central lobe 

 running for some distance 

 up the middle of the fin, 

 and completely covered with 

 scales, while the rays of these 

 fins form a kind of fringe, 

 radiating ou all sides from the 

 central lobe, the skeleton of 

 a fin of this type being shown 

 in Fig. 3. 



From this it will be seen 

 that such a fin consists inter- 

 nally of a long cartilaginous 

 axis, composed of a number of 

 joints (1-9), and that from one 

 or both sides of such joints 

 there are given ofl" obliquely 

 other smaller jointed rods 

 terminating in the fine rays 

 forming the free edges of the fins. How totally diflerent in 

 construction is this kind of fin from that of the perch wiU 

 be manifest from our description, and a comparison of 

 Fig. 3 with Fig. 2. Fishes with paired fins like those of 

 the perch may be well termed fan-finned fishes; while 

 those with fins of the type represented in Fig. 4 may be 

 known as the fringe-finned fishes. 



At the present day the only fringe-finned fishes are the 

 baramunda and the African and American mud-fishes 

 (which, as we have said, are the sole living representatives 

 of the lung-fishes), together with the bichir and an allied 

 species, and the gar-pike, which, as we have seen, belong 

 to the ganoids. All the oldest ganoids, such as those 

 found in the old red sandstone of Scotland, of which an 

 example is figured in the article on "Mail-clad Animals," 



of skull 

 tack-fins 



seen. 



Fig. 3. — Skeleton of pectoral fin 

 of an extinct Shark. (After 

 Fritseh.) 



are, however, likewise of the fringe-finned type ; 

 and since a gradual passage from these primeval 

 ganoids can be traced tbrough later ganoids 

 found in the upper palreozoic and secondary rocks 

 to the bony fishes so characteristic of our present 

 seas and rivers, there can be no manner of doubt 

 but that the fringe-finned type is the most ancient 

 one, which has gradually become modified into the 

 modern fan-finned form. 



The evidence that the fringe-finned type is the 

 r-«s5^Si oldest does not, however, stop here ; for, curiously 

 ** enough, not only had the early ganoids and lung- 



fishes this kind of fin, but the same type likewise 

 obtained in the primeval sharks. The fin-skeleton 

 represented in Fig. 3 belongs indeed to a member 

 of the same group of sharks as does the species of 

 which the entire skeleton is shown in Fig. -1, where 

 the rod-like axis in both pairs of fins is distinctly 

 Now we have already mentioned that modern sharks 

 and rays have fan-like fins ; and it is, therefore, clear that 

 both in the sharks and in the compound group represented 

 by the ganoid and the bony fishes there has been an inde- 

 pendent transition from the fringe-finned to the fan-finned 

 type. On the other hand, in the lung-fishes, which, as we 

 shall see shortly, are a very ancient race, the fringe-iinned 

 structure has been preserved without alteration throughout 

 countless ages. 



\Ye are still unacquainted with the habits of some of the 

 living fringe-finned fishes, but at least the lung-fishes are 

 species living partially buried in the mud, and are evidently 

 not adapted for swimming rapidly. On the other hand, the 

 fan-finned modern fishes, whether they be sharks or 

 whether they be bony fishes, are generally adapted for 

 rapid motion in the water. Any person who has watched 

 a bowl of gold-fish will not have failed to notice the 

 incessant and rapid motion of their film-like fins, and it 

 is quite evident that this rapid motion could only be 

 produced by fins of the fan-like structure. The fringe-fins 

 are, indeed, more like clumsy paddles, capable only of 

 comparatively slow and steady motions ; such movements 

 being sufficient for fishes protected either by the bony 

 armour of the ganoids, or by the spines with which the 

 early sharks (Fig. 4) were armed. The fan-like fin is 

 therefore obviously the most specialized type of structure, 

 and as the ganoids in their advance towards the bony 

 fishes gradually acquired this fan-like fin, and with it, we 

 may presume, increased speed, it was essential that their 

 enemies the sharks should follow suit in order to be able to 

 catch their prey. This would appear to be a sufficient 

 reason for the attainment of the fan-like structure of fin in 

 both these groups of fishes. It is, however, very remark- 

 able that this structure of the fins having been indepen- 

 dently developed in the two groups should have become so 

 alike' as it is. On the other hand, the lung-fishes, 

 together with the gar-pike and the bichir, never having 

 had occasion to abandon the mud-loving and sluggish 

 habits of their palieozoic ancestors, have fortunately 

 preserved for us intact the old fi-inge-finned type of 

 swimming organs. 



It is not, however, in regard to these paired fins alone 

 that fishes show a modification from a long, jointed, axial 

 structure, to one which stops suddenly in an expanded 

 termination, from which arises a fan-shaped arrangement 

 of rays, the same kind of modification, although far less 

 generally, having also taken place among fishes in the 

 structure of their tails. 



Thus, in all the primeval fishes the backbone (as shown 

 in Fig. 4) is continued right to the very end of the tail, 

 where it terminates in a point. On either side of the 



