CHAMBERS'S INFORMATION FOR THE PEOPLE. 



is often an article of value for the fish which it 

 has in its stomach, although its own flesh is but 

 little worth. 



Sub-order (D). The Plectognatlii approach the 

 Cartiligines in many points of organisation ; prin- 

 cipally, however, in the slow ossification of the 

 skeleton, and the imperfect structure of the mouth. 

 They derive their name from the union of the 

 upper jaw to the skull ; so that its motion is ob- 

 tained, not from a distinct joint, but by the mere 

 flexibility of the half-ossified cartilages. The gill- 

 lid is concealed under the thick skin, with only a 

 small opening ; the ribs are scarcely developed ; 

 and there are no true ventral fins. This order 

 contains two families : 



The family of the Sclerodermata contain fishes 

 which are remarkable for their hard and granu- 

 lated skins. They have the head prolonged into 

 a muzzle : at the extremity of this is the mouth, 

 which is armed with a series of distinct teeth. 

 The Ostracion (Trunk-fish), which is found in the 

 Indian and American seas, has the whole of the 

 body except the tail covered with an inflexible 

 suit of bony armour, composed of bony plates. 

 The Batistes (File-fishes) have the dermal skeleton 

 in the form of small grains, leaving the skin a 

 certain amount of flexibility. They are prin- 

 cipally inhabitants of warm seas. 



The Gymnodonta (Naked-toothed fishes) are 

 distinguished by having the jaws covered with an 

 ivory-like substance arranged in small plates, 

 which are reproduced as soon as destroyed by use. 

 They live on Crustacea and sea-weed. The most 

 remarkable species of this family are the Spinous 

 Globe-fishes, Diodon and Tetraodon, which have 

 the power of blowing themselves up like balloons, 

 by filling with air a large sac which nearly sur- 

 rounds the abdomen. They are defended by 

 spines over their whole surface, which are erected 

 as they are inflated. The Sun-fish has a body of 

 somewhat similar form, but incapable of inflation ; 

 from the shortness of its tail, it looks like the 

 anterior half of a fish cut in two in the middle. 

 There are only two British species of sun-fish 

 (Orthagoriscus), and some specimens have been 

 known to weigh 300 pounds. 



Sub-order (E). The Lophobranchii are a small 



Hippocampus brevirostres. 

 group, which have the gills arranged in tufts in 



160 



pairs upon the branchial arches. The swim- 

 bladder is destitute of an air-duct. The body is 

 covered, not with small scales, but with shields or 

 plates, which often give it an angular form. In 

 general, they are of small size, and almost with- 

 out flesh. The Syngnathus (Pipe-fish) possesses 

 a long tubular snout. It is peculiar for the pro- 

 tection it affords to its young. The eggs are 

 conveyed into a sort of pouch under the body 

 of the male, and are hatched there, the young 

 fry afterwards finding their way out. Some of 

 these are found in the British seas ; as are also the 

 Hippocampi, commonly called 'sea-horses,' from 

 the resemblance of the upper part of the body to 

 the head and neck of a horse in miniature. The 

 tail is prehensile, and they climb or hold on to the 

 stalks of marine plants by its means. 



ORDER 4. The Ganoidei are a group of which 

 there are few living representatives at the present 

 day, but they attained an enormous development 

 during the Palaeozoic period, when they were 

 represented by such genera as Pterichthys, Coc- 

 costeus, Pteraspis, and Cephalaspis, from the Old 

 Red Sandstone. In this group of fishes, the 

 endo-skeleton is very imperfectly ossified ; in fact, 

 in many of the fossil forms the skeleton was 

 entirely cartilaginous. The exo-skeleton consists of 

 plates or scales, which are composed of an upper 

 layer resembling enamel, and termed ganoine, 

 and an under layer of true bone. These plates in 

 some genera are of a rhomboidal form, and deli- 

 cately sculptured, and being placed edge to edge, 

 afford an efficient yet flexible investment for the 

 soft parts within. The ventral fins are usually 

 present, but they are placed far back near the 

 anus. In some recent forms, such as the Polyp- 

 terus, and in many extinct forms, the paired fins 

 have their fin-rays arranged round a central lobe. 

 Those fishes with fins so constructed are ' fringe- 

 finned,' and belong to the division Crossopterygidce 

 of Huxley. The tail is generally unequally lobed, 

 when it is said to be ' heterocercal.' The gills 

 have the same structure as in the Teleostei. The 

 swim-bladder possesses an air-duct which com- 

 municates with the pharynx. The bulbus arte- 

 riosus is rhythmically contractile, and is separ- 

 ated from the ventricle by several rows of valves. 

 The intestine is furnished with a folding of its 

 mucous membrane, so as to form a spiral valve. 



The Lepidosteus (Bony Pike) of the rivers and 

 lakes of North America, and the Polypterus of the 

 Nile, are recent examples. The former has the 

 jaws prolonged into a long narrow snout, and the 

 tail heterocercal ; the body is protected by trans- 

 versely arranged rows of ganoid plates. The latter 

 has its dorsal fin broken up into a series of small 

 fins, each of which has a strong spine in front,. 

 and a soft fin attached to it posteriorly. The 

 SturionidcB) or Sturgeons, also belong to this 

 order. In them the notochord is persistent, and 

 the tail is heterocercal. They are found in the 

 Black and Caspian Seas, and ascend the rivers 

 for the purpose of spawning. They are extremely 

 valuable to man, their roe furnishing the caviare 

 so much esteemed in Russia, and the air-bladder 

 furnishing isinglass. The Common Sturgeon 

 (Actipenser sturid) of the British coasts is about 

 six feet long ; but the Beluga (A. huso) of the 

 Caspian Sea measures fifteen feet in length. The 

 Spatularia (Paddle-fish) of North America is a, 

 nearly allied form. 



