122 



CHONDRICHTH YES 



FIG. 82. 

 Raja blanda, Holt, cp, surface of the cartilage, 



Maja oianaa, noic. cp, sunace o 

 showing prismatic calcifications, ct. 



and worn teeth drop off, and are replaced from behind by new 

 ones. 



The teeth, in Elasmobranchs, and indeed in all Chrondrichthyes 

 excepting the Acanthodii, are not firmly fixed to the jaw, but are 

 merely attached to the cartilaginous jaws by connective tissue. 

 They may become much specialised in structure arid disposition, 

 leading to the development of a highly characteristic dentition in 

 various groups. Occasionally several tooth-germs may fuse to form 

 compound teeth. Large specialised denticles may also be found on 

 the body. Among these are to be reckoned the powerful spines, 



which often occur in front 

 of the fins (Markert [291]). 

 In some cases (Acanthias, 

 Fig. 50) these are merely 

 large hollow cones of the 

 ordinary structure, resting on 

 a cartilaginous radial. 



The endoskeleton remains 

 cartilaginous ; but is often 

 strengthened, especially on 

 its surface, by the deposition 

 of calcareous salts, generally 

 in the form of prismatic plates 

 (Fig. 82). In living Chondrichthyes the notochordal fibrous sheath 

 is always invaded by the surrounding mesoblastic layer (p. 99, 

 Fig. 60). The median fin-folds are always more or less subdivided, 

 and the caudal fin usually becomes heterocercal. The vertebral 

 column extends to the extremity of the dorsal lobe. 



Both the median and the paired fins are provided with very 

 numerous horny fin-rays, or ceratotrichia. This is one of the most 

 characteristic features of the group (Mayer [297], Klaatsch [264], 

 Goodrich [175]). The ceratotrichia are slender, unjointed rods of 

 homogeneous fibrous substance secreted by the mesoblastic cells. 

 As a rule, they are very much more numerous than the underlying 

 somactidia. At the growing distal edge of'the fin they are seen to 

 originate immediately below the basement membrane. But later on 

 they sink into the connective tissue, and to their proximal ends 

 are attached the radial muscles of the fins. The ceratotrichia are 

 developed on both sides of the fins, and proximally embrace the 

 ends of the median cartilaginous radials (Fig. 83). 



In all the living Elasmobranchs, except the highly specialised 

 Holocephali, the gill-slits always open to the exterior independently. 

 This was probably also the case in the extinct Chondrichthyes (with 

 the possible exception of the AcanthodiO. Where the gill-septum 

 reaches the surface, a strip of the ordinary denticle-bearing skin 

 passes between the openings (Figs. 26, 57). 



