RAYS AND SHARKS. 



108.") 



BLASMOBRANCHII PLAGIOSTOMI . 



Elasmobranchs with several outer branchial apertures and with more or less distinct' spiracles. Skin com- 

 monly shagreened, with papillae and spines (placoid scales). Notochord constricted by the formation of ver- 

 tebrae. Palatoquadrate part mobile independently of the rest of the skull and dentigerous. Paired fins 



inserted horizontally. 



That course of evolution from the Paheidiihijcs 

 which i.s re|ireseiit(!(l in our times by the Plagiostoms, 

 the sul)oi'der o'i tlie Sliarks and Kays, has advanced so 

 far, as regards the structure of the spinal column, tliat 

 the most highly- developed forms possess complete ver- 

 tebra-, fully divided from each other, and retaining the 

 notochord imly in the intervertebral spaces between the 

 centra, which are am[ihicoelous or conically hollowed at 

 both ends. In theii- typical forms the two well-known 

 phalanxes — the Sharks, with their elongated, fusiform 

 or clavate Ixidy, and the Rays, wdiose pectoral fins, ex- 

 panded and coalescent with the sides of the body, give 

 the forepart (the head and trunk) a discoidal shape — 

 are even externally so unlike each other that no other 

 character than the outer shajie seems necessary, espe- 

 cially in the case of the Scandinavian fauna. f5ut inter- 

 mediate forms, Itays with less expanded pectoral fins 

 or with an unusually terete and thick tail, and Sharks 

 with triangular, wide-based pectoral fins and a body 

 more than commonly depressed, occur in foreign seas; 

 and during youth — in a larval stage when the Pla- 

 giostoras are still furnished with filamentous external 

 gills — the Rays have a gi-eat external resemblance to 

 Sharks. The latter were undoubtedly the earlier forms, 

 geologically speaking, and in the present age the most 

 imperfectly developed forms — as Hasse in particular 

 has pointed out with respect to the structure of the 

 spinal column — are Sharks. 



Even from the Silurian deposits ichthyodorulites 

 are known which have l)een referred to a Selachoid 

 genus (Oiuhiis), and in the Carboniferous seas Sharks 

 were very numerous: — the family of the Cestraciontkhe, 

 which still survives in the Pacific, already existed. And 

 ever since the Jurassic Period the Notidanoid family 

 has survived with a structure which in the tropical 

 and subtropical seas of the present age represents 



the lowest stages in the evolution of the Plagiosto- 

 mous type. 



Individually too most of the Plagiostonis show 

 great tenacity of life. During the fishery for tiie (Green- 

 land Shark in the Arctic Ocean bcitween Norway and 

 Bear Island, these large fishes are hauled up from a 

 depth of a hundred fathoms or so and deposited on the 

 deck of the. fishing-vessel. There they lie motionless, 

 partly, no doubt, owing to their sluggish temperament, 

 but probably stupefied as well by the sudden reduction 

 of the pressure in which they are accustomed to live. 

 Their belly is openeil witli a large knife, and the liver 

 removed for the sake of the oil which it contains; but 

 nothing is done with the rest of the body, unless a 

 fresh bait be recjuired for the huge hook. A Greenland 

 Shark in this condition may be skinned and entirely 

 disembowelled; but the manifestations (jf life do not 

 cease for many hours. Even after the head lias been 

 cut off from the body and has lain some hours by 

 itself, it is dangerous to get one's fingers between the 

 jaws, for they may easily be bitten off, and the bite is 

 so tenacious that one may attempt in vain to extricate 

 what the jaws have once grasped. Couch tells an anecdote 

 of a Blue Shark which had been hooked and deprived 

 of its liver in a similar manner. With the entrails 

 hanging out of the belly it was restored to the water 

 and followed the fishing-vessel for some time. It was 

 not long before the fish tried to seize a Mackerel that 

 had dropi)ed from the net of the vessel. On another 

 occasion a Shark was thrown overboard after being 

 decapitated. For some hours the fish kept swimming 

 round about — as a boy on board expressed himself — 

 as if the body was looking for its head. Of the common 

 Skate the same author relates that its heart may be cut 

 out and retain contractile powers for at least 25 hours, 

 when he saw the auricle beating five times a minute. 



" TtXd'/lOg, slantinij, obliijue, and acdf-ici, mouth. 



' Though sometimes obliterated. 



' The Electric Rays and yomg Eagle-Hays, with smooth skin, are exceptions. 



