318 THE SEA SHORE 



is lost as the rays penetrate into deep water, so that the bottoms 

 of deep seas must be more or less darkened. To allow for this 

 loss, we find that the species living at moderate depths are pro- 

 vided with larger eyes to enable them to see their prey and their 

 mates ; but at still greater depths, where the sun's light cannot 

 penetrate, the fishes are either blind, or are possessed of luminous 

 organs which enable them to see their way. Again, as the sea is 

 so thinly populated at such great depths, the carnivorous species 

 do not find abundant food always at hand, hence they are often 

 provided with such mouths and stomachs as will allow them to 

 make the best of favourable opportunities, some being capable of 

 swallowing a fish quite as large as themselves. 



We often find fishes roughly classified into fresh-water and salt- 

 water species, and although such a division is at times convenient, 

 it must be remembered that some of the former migrate into 

 brackish and even into salt water, while some of the latter ascend 

 estuaries and rivers either for the purpose of obtaining suitable 

 food, or for the deposition of their eggs. 



The fishes that frequent our coasts may be classified into two 

 main groups, those with cartilaginous skeletons (Elasmobranchii), 

 and the bony fishes (Teleostomi). Both these are divided into 

 family groups, and we shall deal more or less briefly with all the 

 important families that include common British marine fishes, but 

 giving more attention to those species that are truly littoral in 

 habit species that may be found in the rock pools or under stones 

 at low tide, and which may be obtained by the amateur angler 

 working from rocks, piers, &c. 



The cartilaginous fishes include the Sharks, Dogfishes, and 

 Rays. They have pouchlike gills, five or more on each side, each 

 one opening to the exterior by a separate slit. The skin generally 

 contains bony elements that are toothlike in structure and often in 

 form ; the mouth is usually on the under side of the head, and the 

 tail is nearly always of the heterocercal kind. They are all 

 carnivorous creatures, and often exceedingly voracious ; and are 

 represented in our seas by the Rays and Dogfishes. 



Rays or Skates (family Raiidcz), of which there are six or seven 

 British species, are readily known by their broad flattened rhorn- 

 boidal bodies, with the mouth on the under side of the head, a 

 longitudinal fold on each side of the tail, and pectoral fins extending 

 quite or nearly to the front point of the head. 



Two of these fishes are very common in our markets, one being 



