CHARACTERISTICS OF VALUABLE MARINE FISHES 37 



together in the fishes of the higher orders, with which we are 

 most familiar, and which include the kinds to be chiefly con- 

 sidered in the present work. There are three orders which 

 are of no importance commercially in this country. One 

 of these may be called the lung-fishes, because although 

 they have gills and fins they are provided with primitive lungs, 

 corresponding to the air-bladder of other fishes, and with 

 these to a limited extent they breathe air. None of these 

 are European. Another order contains the lampreys and 

 the curious hag-fish or borer which is familiar enough to line 

 fishermen of the north-east coast of Britain. These have no 

 paired fins. Their mouth is suctorial, and their gill-passages 

 instead of being wide slits are in the form of pouches which 

 open on the sides of the neck by small holes or by tubes leading 

 to a single hole. A third order contains only a singular and 

 much simplified creature called the lancelet ox AmpJiioxus . The 

 structure of this creature is shown in Fig. 2. It has no side fins 

 no bones, no gristle, and properly speaking no head. It has no 

 eyes, hearing organs or smelling organs like those of other fishes, 

 but a single rudimentary eye and one simple nostril : it has no 

 distinct brain. But still it has two of the most essential organs 

 of a fish : a rudiment of a back-bone, and gill-slits. The former 

 is a rod of laminated tissue running from one end of the body to 

 the other, the latter are very numerous slits in the sides of the 

 gullet, but they open into a gill chamber which communicates 

 with the exterior by a single small aperture. The lancelet is 

 common enough on our southern shores. It is only two inches long, 

 is very transparent, and buries itself in the sand. We shall not need 

 to consider these three orders any further in the present work. 



Among the fishes which have a commerical value are 

 members of two very distinct orders, dog-fishes and skates or 

 rays on the one hand, and scaly fishes, such as the herring, 

 the plaice, and the cod on the other. In the first order the 

 skeleton contains no true bone, but consists of cartilage, or 

 gristle which is soft and elastic. The skin is furnished not with 

 scales, but with bony spines, plates, or tubercles which contain 

 lime, and are in consequence hard and strong. The arrange- 

 ment of the gills is very different from that seen in scaly fishes. 

 There is no gill-cover, but five separate gill-slits open on the 

 surface of the skin in the neck region, and can be seen to 



