522 



SHARKS AND RAYS. 



between the superior arches of the vertebrae. The eggs are generally invested in 

 horny rhomboidal capsules, furnished at the four corners with long tendril-like 

 filaments, by which they attach themselves to the stems of seaweeds and other 

 bodies, as shown in the figure of the lesser spotted dog-fish given on p. 529. In 

 some species, however, the eggs are hatched within the body of the female ; and 

 in all cases the embryos are furnished with external gills, which are shed before 

 birth. All the members of the order subsist on animal substances, but whereas the 

 typical sharks are highly predaceous creatures, seizing and devouring everything 

 they come across, some of the largest species are armed only with small teeth, and 

 feed on molluscs and other invertebrates. The rays, too, are largely shell-fish 

 eaters, and most of them differ from the sharks in living on or near the bottom, 

 instead of swimming about actively at or just below the surface. All the species 

 are typically marine, but many ascend tidal rivers, and in the Viti Levu Lake in 

 Fiji, as well as in the Nicaragua Lake in South America, there are sharks dwelling 

 permanently in fresh water. The species inhabiting the former lake, which is cut 

 off* from the sea by a cataract, is Carcharias gangeticus, common alike in the 

 Ganges and in the Tigris, and ascending in the latter river to a distance of three 

 hundred and fifty miles from the sea in a straight line. Then, again, a species of 

 saw-fish is found in a fresh-water lake in the Philippines. It has been commonly 

 stated that sharks have the power of scenting their prey from a distance, since 

 they rapidly congregate whenever animal refuse or other decomposing matter is 

 cast overboard from a ship ; but it may be suggested that such assemblages, as in 

 the case of vultures, are rather due to one shark following the movements of 

 another, and thus being attracted to the central point. The order was formerly 

 divided into two subordinal groups, based upon the conformation of the body ; 

 the one group including all the sharks and dog-fishes, and the other the rays and 

 their immediate allies. It has been found, however, that although this difference 

 in bodily form is of considerable importance in classification, yet that it does not 

 constitute the essential line of distinction, which is based upon a difference in the 

 internal structure of the bodies of the vertebra?. Taking this character as a basis, 

 the members of the order may be arranged in two subordinal groups, the first of 

 which comprises the true sharks and dog-fishes, while the second includes the 

 spiny dog-fishes, saw-fishes, eagle-rays, and rays. 



The Blue Shark and its Allies, — Family Caechaeiid^j. 



The well-known blue shark (Carcharias glaucus), of which examples are 

 depicted in our coloured Plate, may be taken as the typical representative of the 

 leading family of the first suborder. Before, however, indicating the characters 

 of the family, we must refer to those of the suborder, for which the name of 

 Asterospondyli has been suggested. The essential feature of this group is to be 

 found in the circumstance that when the bodies of the vertebras are fully calcified, 

 the radiating plates in the interior predominate over the circular ones, so that a 

 transverse section presents a star-like arrangement. All these fishes have an anal 

 fin, and the form of the body elongated and subcylindrical, while the tail is 

 powerful and well adapted for swimming. In no case are the pectoral fins 



