62 Memoir Sears Foundation for Marine Research 



Class CHONDRICHTHYES 



Elasmobranchs and Chimaeroids 



Characters. Fish-like vertebrates with well developed lower jaws and bony teeth; 

 2 pairs of appendages supported by pectoral and pelvic girdles; a cartilaginous skeleton 

 which, while more or less calcified, lacks any true bone; scales essentially tooth-like in 

 structure, the ectoderm as well as the mesoderm sharing in their formation (placoid 

 scales); two nostrils, each single, partially subdivided; olfactory sacs blind, not opening 

 into mouth; posterior end of vertebral column either straight or heterocercal ; sympathetic 

 nervous system, pancreas, spleen and contractile arterial cone present; two, three or more 

 series of heart valves; swim bladder absent. 



Relation to Other Classes. Chondrichthyes are most obviously separated from the 

 Cyclostomes by their well developed lower jaws and bony teeth, by their much more 

 highly developed cranium and visceral skeleton, as well as by the presence of pectoral and 

 pelvic girdles, paired limbs, spleen and a contractile conus arteriosus with two, three 

 or more series of heart valves. The lack of true bone in the skeleton, which is most 

 apparent in the skull and pectoral girdle, separates them from all so-called higher fishes, 

 including the Lung Fishes (Dipnoi). Other features marking them apart from bony fishes 

 are: (a) cranium, without sutures consequent on its lack of bone; (b) outer margins of fins 

 supported by horny rays or filaments as contrasted with bony rays or spines among bony 

 fishes; (c) first gill pouch with a row of gill filaments, which are lacking among bony fishes, 

 and gill filaments attached to the interbranchial septa except at the tips (free for a greater 

 or lesser part of their length in bony fishes) ; (d) no true operculum, but at most a fold 

 of skin serving the same purpose (in Chimaeroids); (e) nostrils single; (f) teeth simply 

 imbedded in the gums, not firmly attached to jaws or imbedded in the latter; (g) scales 

 (placoid or dermal denticles) tooth-like in structure, consisting of a hollow cone of 

 dentine of dermal origin surrounding a pulp cavity; externally this cone is covered with a 

 layer of an enamel-like substance (vitro-dentine) or possibly true enamel (among Rays) 

 formed at least partially by the epidermis; i.e., it is of ectodermal origin,''' whereas in bony 

 fishes the scales are formed by the dermis alone, i.e., they are bone-like in origin; (h) ferti- 

 lization is internal, and in all modern representatives is effected by cartilaginous appen- 

 dages, commonly called claspers, which are developed from the inner margins of the pelvic 

 fins of the males (among such of the bony fishes as have internal fertilization, the intromit- 

 tent organs are developed either from the genital papilla or urogenital orifice, or in connec- 

 tion with the anal fin, or as a special structure situated on the chest, but never from the pelvic 

 fins). Furthermore, the invariable presence among the Chondrichthyes of the so-called 

 spiral valve in the posterior portion of the intestine separates them from most bony 

 fishes," as does the presence of a pair of spiracles in many of them (representing vestigial 

 gill clefts) which open on the dorsal or dorso-lateral side of the head, frequently with a 



lb. See Tomes (Philos. Trans., igo, 1898: 460) for further discussion of this question. 

 1. In a vestigial form in sturgeons, Amia, lung fishes and some others. 



