FISHES OF THE GULF OF MAINE 



15 



CARTILAGINOUS FISHES. CLASS CHONDRICHTHYES 



The Shark and Skate Tribes, and the Chimaeroids 



These are fishlike vertebrates with well-devel- 

 oped fins and teeth, and with 2 pairs of fins, one of 

 them supported by the pectoral girdle, the other 

 by the pelvic girdle. Their most distinctive char- 

 acter, as contrasted with the bony fishes (p. 80) 

 is that their entire skeleton, including the skull, 

 is cartilaginous, without any true bone, though it 

 is partly calcified, especially in the vertebrae; 

 the skull is far simpler than it is among the bony 

 fishes; the gill filaments are attached throughout 

 their lengths to the partitions between the gill 

 openings instead of being free; and the rear portion 



of the digestive tract is modified into the so-called 

 "spiral valve" by the development of a special fold 

 from its lining layer, which only a few bony fishes 

 have. 



Fertilization is internal in all of them, and is 

 effected by a pair of rodlike copulatory organs, each 

 of which is developed from the inner edge of one of 

 the two pelvic fins, and is supported by one or more 

 cartilages. 



The sharks and rays are usually looked upon as 

 more primitive than the bony fishes. 



SHARKS, TORPEDOES, SKATES, AND RAYS. SUBCLASS ELASMOBRANCHII 



The most obvious external character by which 

 all the sharks, skates, and rays are distinguishable 

 from all of the bony fishes is that they have five or 

 more gill openings on either side of the head, in- 

 stead of only one. They recall the lampreys in 

 this respect, but it is a commonplace that their 

 jaws and teeth are extremely well-developed. 

 Their skins are tough, and are studded in most of 

 them with denticles (placoid scales), which are not 

 homologous with the scales of bony fishes, for both 

 dermis and epidermis take part in their formation, 

 instead of the dermis alone. The teeth of the 

 sharks and rays represent placoid scales that are 

 modified and are embedded in the gums alone, not 

 in the jaws. The fins are supported at their bases 

 by segmented cartilaginous rods, supplemented in 

 all of the sharks, and in some of the rays by nu- 

 merous slender horny fibers further out, instead of 

 by rays or spines of the sorts that are seen in the 

 bony fishes. All of their fins are covered with the 

 same leathery skin that clothes the body. Among 

 sharks the tail is uneven ("heterocercal"), with 



the vertebral column extending out into its upper 

 lobe, but it is whip-like in most of the skates and 

 rays, with no definite caudal fin. The torpedo 

 is an exception to this rule. 



The modern representatives of the subclass may 

 be grouped in two orders, the one (Selachii) to 

 include all living sharks, the other (Batoidei) to 

 include the sawfishes, the skates and the rays. 

 They are separated one from the other by the 

 following external differences, and there are skele- 

 tal differences between them as well: 26 



1 . The gill openings are at least partly on the sides; the 

 edges of the pectoral fins are not attached to the sides of the 

 head in front of the gill openings; the upper edges of the 

 orbits are free from the eyeballs, so that they form free 



eyelids Sharks, (p. 15). 



The gill openings are entirely on the lower surface; the 

 edges of the pectoral fins are attached to the side of the 

 head in front of the gill openings; the upper edges of the 

 orbits are attached to the eyeballs so that they do not form 

 free eyelids Sawfishes, skates and rays, (p. 57). 



*• For further discussion, s.t Blgelov and Schroeder, Fishes Western 

 North Atlantic, Pt. 1, ch. :i, 1948. p M 



Sharks. Order Selachii 



Sharks always are objects of interest, not only 

 to fishermen and mariners but to seaside visitors 

 generally, because of their evil appearance, their 

 ferocity, the large size to which some of them grow, 

 the destruction they wreak on fishermen's nets 

 and lines as well as on the smaller fishes on which 

 they prey, and because of the bad reputation 

 certain kinds have earned as maneaters. 



The Gulf of Maine is not particularly rich in 

 sharks (very poor indeed compared with our 

 southern coasts), for while the number of species 

 actually recorded there is considerable (indeed 

 any high-seas shark might stray thither) the little 

 spiny dogfish alone is numerous in the sense in 

 which this term is applied to the various com- 

 mercial fishes. And only two of the larger species, 



