s^ c>^o 



9 ^ ^ 



lateral-line canal 



Figure 8-42. Anterior end of the developing scale rows of a 70-mm specimen of tepisosteus. 



General observations Practically all the types of hard tis- 

 sues are found developed already in the Ordovician agnaths. 

 There was an enamel-like tissue {Aslraspis) and a dentine 

 of two types: ordinary coarse-tubed dentine in Eriptychius, 

 agreeing with that in ostracoderms and elasmobranchs, and 

 a more fine-tubed dentine in Astraspis, similar to that in 

 many higher fishes. Bone tissue without enclosed cells, simi- 

 lar to the aspiden of the later heterostracans, was present in 

 Astraspis and, of a somewhat different type, in Enptychtus. 

 That of Aslraspis was coarsely fibrous with well-developed 

 osteons. In Eriptychius, several generations of denticles, one 

 superimposed over the other, occurred on some surfaces. 



In contrast to the heterostracans, the osteostracans, as the 

 name implies, had true cellular bone. This same variation 

 is observed in acanthodians and sharks — with cellular or 

 acellular bone forming the bases of the scales. In the acti- 

 nopterygians, an acellular bone is developed in the teleost 

 and in the early palaeoniscoids, Onnkuina. The latter is thus 

 a suspect palaeoniscoid, but other palaeoniscoids (Scanilepis) 

 show tendencies in the direction of acellularity. 



A possible conclusion is that there are several closely 

 interrelated scale types: placoid, agnath, arthrodire, ganoid, 

 and cosmoid. The primitive condition of the last four in- 

 volved the early appearance of denticles (placoid scales) 

 followed by the progressive development of a bony base for 

 these. Differences in the ontogeny (developmental stages) 

 and histology can be used to identify the several types. 



The various hard materials observed, enamel, dentine, 

 and bone, or the intergrading substances, all appear to be 

 related and to intergrade. 



FIN RAYS 



The fins of many fishes are supported by fin rays, formed 

 of bilateral components (Figure 8-46). Some rays are jointed 

 and appear to be formed of modified scales (Figure 8-44). 

 In the cyclostomes, there are no scales and also no fin rays, 

 but in fossil agnaths scales were present. Both living and 



fossil agnaths had radials, extensions from the neural and 

 hemal arches into the caudal fin lobes. In Birkenia, an anas- 

 pid, there were rows of scales forming lepidotrichia on 

 either side of each radial (Figure 5-28). In the cephalaspids, 

 rows of rectangular scales paralleled the radials. 



The primitive sharks were like the anaspids in having the 

 radials extending nearly, or quite, to the margin of the fin — 

 this is the plesodic type of fin characteristic of the rays (Fig- 

 ure 8-44). In the selachian fin, the radials are limited to the 

 basal half and several layers of horny rods or ceratotrichia 

 overlap the radials and extend out to the fin margin — this 

 is an aplesodic fin. The ceratotrichia lie in the dermis; the 

 overlying skin is studded with placoid scales. 



Acanthodians had an aplesodic fin with the radials 

 scarcely extending into the fin. The membrane was sup- 

 ported by ceratotrichia and the skin studded with small 

 scales (Figure 6-68). The ceratotrichia are described as gen- 

 erally unjointed cylindrical rods of uniform diameter, ossi- 

 fied in the proximal part. 



The pectoral fins of the arthrodires are plesodic with a 

 broad base. The caudal fin of Botkriolepis was supported by 

 pairs of calcified rods, which seem to be intermediate in 

 nature between the ceratotrichia of the shark and the radials 

 of the agnath (Figure 8-45). These rods have been identi- 

 fied as radials. 



Among the choanate fishes, the fins of the dipnoans are 

 the most primitive in that they are covered by scales (Fig- 

 ure 8-45). Within the fin of Diplerus, there are radials and 

 bony lepidotrichia. The lepidotrichia are jointed and 

 branched; basally they overlap the radials and distally ex- 

 tend to the margin of the fin. They are grooved on their 

 inner aspect. The basal half of the fin is covered externally 

 by cycloid scales, which grade into rows of rectangular scales 

 paralleling the lepidotrichia. In Neoceralodus or Scaumenacia 

 (Devonian), the scales covering the fins do not form distinct 

 rows. 



The ceratotrichia of Neoceralodus and Proloplerus are 

 slender pliable rods, irregularly jointed, and generally 



240 • THE SKIN AND ITS DERIVATIVES 



