204 INTRODUCTION TO THE FISHES 



shown that it was sandy and that certain molluscs upon which the 

 plaice feed were in abundance. An examination of the recap- 

 tured marked fish fully demonstrated the very great advantage 

 derived by the fish that had been transplanted to the Dogger 

 Bank, as compared with those liberated on the coastal grounds. 

 The small fish transplanted to the Dogger in April showed an 

 average increase of 62 per cent, in length and 360 per cent, in 

 weight by the following mid-winter ; while the fish left to grow on 

 their Usual grounds from April until mid- winter increased only 1 6 

 per cent, in length and 59 per cent, in weight. Thus the growth 

 of the transplanted fish, compared with that of the fish left on the 

 coastal grounds, was four times as great in length, and six times 

 as great in weight. The photograph facing page 235 shows the size 

 of the fish at the time of marking, and one recaptured from the 

 Dogger and one from the coastal grounds. 



In their general structure fishes are well distinguished from other 

 animals, and their most distinctive structures are their fins, gills 

 and air-bladder. The notochord is converted into firm, granular 

 cartilage, sheathed in bone, and divided into segments by bone 

 deposited in its substance. It extends forward along the base 

 of the skull, and develops two oblong convex surfaces, termed the 

 occipital condyles, by which the back of the skull unites with the 

 first vertebra. This mode of union of the skull with the verte- 

 bral column is characteristic of amphibians and mammals, and 

 since the other vertebrates have the skull united to the vertebral 

 column by a single occipital condyle, we have here a link in the 

 chain of evolution from fish-like ancestors, for the Amphibia 

 possess, when they commence their existence, many of the struc- 

 tures of fishes. The skeleton of lepidosiren, one of the curious 

 lung-breathing fishes, presents the closest resemblance to that of 

 the lowest class of reptiles. In amphioxus the cranium is repre- 

 sented merely as a forward continuation of the notochord, and 

 gives no conception of the skull as seen in the fishes ; yet a jointed, 

 cartilaginous arch extends downward round the region of the 

 mouth, and is the foreshadowing of the arch, which is more per- 

 fectly developed around the mouth in the lampreys and hag- 

 fishes. In simplicity of skull-structure the sharks and rays are 

 the next step in the upward series, but there are many points 

 which place these animals among the highest types of fishes, and 



