166 



KNOWLEDGE. 



[September 1, 1891. 



the fish is propelled mainly by the muscular tail with its 

 terminal fin. The fins on the body act mainly as balances, 

 although aidiu!,' to a certain extent in propulsion. These 

 body-fins in all fishes are of two types, namely — paired 

 and median. The number of paired fins is two, the 

 front pair corresponding with the fore limbs, and the 

 hinder pair with the hind limbs of quadrupeds. In the 

 Perch (Fig. 3), the front or pectoral pair of fins are seen 

 immediately behind the head ; the second or pelvic pair 

 being placed below and slightly behind the pectoral ones. 

 In many other fishes (as in Fig. 4) the hinder pair of fins 



Fig. 4. — An Extixct Gaxoid I'lsn. 



occupy, however, a position corresponding with that of the 

 hind limbs of quadrupeds. The pectoral fins, although 

 assisting to a certain extent in the motion of the fish 

 through the water, act rather in directing its course than 

 as propellers. Their chief function is, however, to main- 

 tain the balance of the body Ln the water ; a fish which has 

 lost one of these fins falling over to the opposite side. It 

 will be observed fi'om Fig. 3 that the pectoral fin of a 

 Perch (as well as of most of our existing fishes) consists of 

 a number of rays spreading out in a fan-like manner from 

 the point of attachment to the body. A totally different 

 arrangement obtains, however, in the pectoral fin of the 

 extinct fish represented in Fig. 4. Here it will be seen 

 that the fin consists of a central lobe covered with scales, 

 from the edges of which the fin-rays project as a deep 

 fringe. This more primitive type of fin is indeed very 

 common among the extinct fishes of the Palaeozoic rocks, 

 and still persists in the Barramunda of the Queensland 

 rivers, a figure of which was given in the article on " Mail- 

 Clad Animals." The more important median fins are the 

 dorsal on the back, and the anal in front of the tail. 

 Ln many fishes (Figs. 3 and 4) there are two dorsal 

 fins, one in front of the other ; the fi'ont one being often 

 large and spiny, and the hind one small and soft (Fig. 3). 



The tail and tail-fin form, as we have said, the chief 

 propeller of the fish ; and it will be particularly noticed 

 that the position of this fin is vertical. In swimming, as 

 we may observe in an aquarium where fish are kept, the 

 tail is rapidly and strongly bent fi-om side to side, while 

 the two lobes of its fin have an undulating motion, and 

 thus act like the blades of a screw-propeller. A difierence 

 between the structure of the tail-fin in the two figured 

 fishes recalls the one already noticed in the pectoral fin. 

 Thus in the Perch (Fig. 3) the scaly part of the tail ends in 

 an abrupt and almost straight edge, from which the rays 

 of the tin form a nearly symmetrical fork. In Fig. 4, on 

 the other hand, the scaled part of the tail is produced to a 

 point, extending far back among the fin-rays, which are 

 arranged unsymmetrically along its two edges. It is this 

 latter mode of arrangement which is the older and more 

 primitive. 



In certain fishes which depart more or less widely from 

 the ordinary form there is a corresponding modification in 

 the shape and functions of the fins. For instance, the 

 Rays swim almost entirely by the aid of the greatly ex- 

 panded pectoral fins, which have an undulating motion 

 very similar to that of the median fins of ordinary fishes. 

 On the other hand, in the Fh-ing Fishes (see " Flying 



Animals," Fig. 2) the pectoral fins are enormously elon- 

 gated, so as to act as organs of spurious flight. Again, 

 snake-like fishes, as the Eel, swim by lateral curvatures 

 of the body, in the so-called serpentine manner. 



Tlie only other Vertebrate animals which breathe by 

 means of gills, and can therefore be regarded as primi- 

 tively aquatic, are the young, or larvfc, of the Amphibians 

 (Frogs, &c.). The young Tadpole, as we all know, is an 

 ugly, large-headed creature, swimming by means of lateral 

 movements of its taU. This tail has a vertical fin-like 

 expansion, differing, however, fi-om the fins of fishes by 

 the absence of the bony or car- 

 tilaginous rays found in the 

 latter. We have already alluded 

 to the remarkable metamorphosis 

 undergone by the Tadpole, in the 

 -- course of which the tail is lost, 



the gills are replaced by lungs, 

 and the limbs developed. The 

 adult Frog is an instance of an 

 animal adapted to live partly on 

 land and partly in the water, swimming powerfully in the 

 latter element by the strokes of its long hind legs, of which 

 the toes are fully webbed. The Tailed Amphibians, such as 

 the Newts and Salamanders, are less specially modified 

 than the Frogs, and may be completely aquatic. All the 

 Newts and Salamanders, including the purely aquatic Giant 

 Salamander of Japan, lose, however, their gills in the adult 

 state ; but these are permanently retained in the curious 

 blind Proteus of the caverns of Carniola. 



Among the true reptiles of the present day (all of which 

 breathe by means of hmgs dm'ing the whole of their 

 existence) there are three groups among which aquatic 

 forms occur. The first of these includes the Crocodiles 

 and Alligators, which swim by means of their long tail 

 and limbs. Although thoroughly at home in the water, 

 where they spend a large portion of their time, the organi- 

 zation of these animals has not been so modified for the 

 exigencies of an aquatic life as to depart to any great 

 extent from the normal type. The same remark will apply 

 with still more force to our Common Snake, which is an 

 expert swimmer. In the Sea-Snakes, however, which 

 pass the whole of their life in the tropical seas, the tail 

 assumes a vertically compressed and paddle-like form, and 

 is thus as efficient a propeller as the taU of a fish. These 

 snakes always swim on the surface of the sea, but it is 

 very doubtful if they can have given rise to the stories of 

 the Sea- Serpent. 



Among the Chelonians the Marine Turtles have been 

 especially adapted for an aquatic life by the modification 

 of their limbs into oar-like paddles ; although it is quite 

 clear that this structure is an acquired one. The Soft 

 Turtles (Trionyces), of the rivers of the warmer regions of 

 the globe, are almost equally good swimmers, although 

 their feet, with the exception of being webbed, retain the 

 ordinary type of structure. The Pond-Tortoise, now 

 restricted to Southern Europe, although occurring in the 

 superficial deposits of this country, is almost equally 

 aquatic. Indeed all Tortoises (except perhaps the 

 gigantic ones of the Galapagos and Mascarene Islands) 

 are excellent swimmers, and thus aflbrd a good instance 

 of how some members of a group have gi-adually adapted 

 themselves to an almost completely aquatic life. 



If, however, the Turtles have been specially modified for 

 an aquatic existence, still more markedly is this the case 

 vnth the extinct Ichthyosaurs or Fish-Lizards (Fig. 5). 

 Since the structure of these reptiles has been fuUy 

 noticed in a separate article, we need only allude here 

 to the peculiar pavement-like structure of the bones of 



