1 84 



FISHES 



CHAP. 



fl- 



pointed, enamel-tipped spine of dentine which protrudes through 

 the epidermis, and projects baclcwards on the surface of the body, 

 but is firmly fixed in the skin by the basal plate with which it 

 is continuous. The centre of the under surface of the basal plate 

 is perforated for the entrance of the blood-vessels which pass to 

 the cellular pulp in .the axis of the spine. In the adult Fish 

 the denticles form a fairly close-set covering to the whole body, 

 including the head and even the surfaces of the fins, and are 

 larger on the dorsal than on the ventral surface (Fig. 99). In 

 the Eays {Raia) they are more sparsely scattered, and in different 

 parts of the body may form spines of considerable size for offen- 



FlG. 99. — Surface view 

 of the dermal den- 

 ticles of Scyllmm 

 sp., showing their 

 arrangement in 

 oblique transverse 

 rows, b. Basal plate ; 

 c, canal which per- 

 forates the basal 

 plate and becomes 

 the axial pulp-cavity 

 of the spine ; /.&, 

 intersecting fibrous 

 bands of the dermis ; 

 c>, spine ; in the 

 spine of one scale the 

 dentinal tubules are 

 shown. The smaller 

 denticles are those 

 most recently 

 formed. (After 

 Klaatsch.) 



sive or defensive purposes. The spines vary greatly in shape in 

 different members of the group, sometimes being acutely pointed, 

 and sometimes flattened or depressed, and often they are furnished 

 with smaller accessory spines developed at their bases or from 

 the surface of the basal plate. An arrangement of the denticles 

 in oblique transverse rows is observable in some genera (e.g. 

 Scyllium). In the Saw -Fishes (e.g. Pristis) the denticles 

 which fringe the lateral margins of the long flattened rostrum 

 are not only enormously enlarged, but are implanted in sockets 

 and form the teeth of the saw (Fig. 262). In the Holocephali 

 the smooth skin is almost entirely devoid of exoskeletal struc- 

 tures, but dermal denticles are present on the frontal and anterior 

 claspers, and in the young there may be a double row of small 

 denticles along the back. 



