THE ELASMOBRANCII FISHES 



:33 



MODIFICATION OF SCALES 



By a peculiar liypertropliy modifications may arise wliich, altliough essen- 

 tially like the primitive shagreen denticles in strnctnre, greatly differ from 

 them in form. Such hypertrophy may result in the production of a fin spine 

 like that in Heterodontus (fig. 88) and the Spinacidae; a tooth like that in the 

 sawfishes, Pristis (fig. 38) and Pristiophorus; a sting like that in the sting rays 

 (fig. 42) ; or it may result in other variously modified structures, such for ex- 

 ample as the branchial rakers in Cetorhmus (fig. 44) and 

 Ellin don. 



Fin Spine 



In some of the types in which fin spines are present they 

 are so rudimentary as to be but little larger than enlarged 

 scales, as exemplified in Cenfroscymnus. In others, as in 

 Heterodontus (fig. 88) and Acanthias, they are pronounced 

 structures. In general, they are located just anterior to the 

 dorsal fins, the posterior one (fig. 36) being longer than the 

 anterior. For almost half its length the spine is buried in 

 the integument. The buried part is designated as the root 

 or base and the exposed portion the crown or spine proper. 



If such a structure be removed and more closely studied, 

 its deeply imbedded base is seen to be triangular in shape. 

 The spine contains a large central cavity which when in 

 place fits over a cartilage of the fin skeleton. The walls of 

 the spine are made of dentine which in the crown consists 

 of a double layer. The more superficial layer is bounded 

 anteriorly and laterally by a layer of enamel, but enamel 

 does not extend over the posterior groove which fits close 

 up against the basal cartilage of the fin skeleton. A more or less compact laj^er 

 of pigment {pg., fig. 40) separates the enamel (e.) in front from the layer of 

 dentine (d.). 



The development of such a fin spine is of considerable interest. Figure 37a 

 represents a sagittal section of an early stage of Acanthias in which a mass of 

 the epidermis (ep.) has sunk into the dermis, just in front of the dorsal fin. 

 The bounding layer of this section becomes the enamel organ (e.o.). The for- 

 mation of the enamel of the spine, and of a part of the dentine, is singularly 

 modified by the peculiar position of this organ. It will be observed that the 

 enamel organ here covers only the anterior upper part and sides of the de- 

 veloping spine, instead of forming a cap over the entire structure as it does in 

 a placoid scale. As a result enamel is present only on the front and sides of the 

 crown, little being produced posteriorly {e., fig. 40). The odontoblasts {od., 

 fig. 37a) just under the enamel organ lay down dentine so that these two layers 

 so far as they go are like the enamel and dentine of a common placoid scale, 

 but tlie greater mass of dentine is formed back of tliis dentine. 



Fig. 36. Second fin 



spine, Squahts 



sucMii. 



