lepidotrichia 



GYROPTYCHIUS 



B POLYPTERUS 



n DTrrmm i 



^^mmnxmnnin;^^ 



rrmn 



RAJA LINTEA 



SQUALUS 



5 ceratotrichia 



Figure 8-44. Fin rays and radiols as observed in a choonate, on actinopterygian, and two 

 chondrichthions. (After Jorvik, 1959) 



outward through the body cover, thus exposing the lepido- 

 trichia as the outermost layer of scales. 



In the primitive actinopterygian fish, there is sometimes 

 a gradual transition from body scale to lepidotrichium just 

 as in the primitive choanate. The lepidotrichium is bony, 

 jointed, branched, and guttered on its inner surface. Its basal 

 part e.xtends into the body to overlap the radial. The num- 

 ber of fin rays, or lepidotrichia, was usually greater than 

 the number of radials, but in advanced forms these were 

 generally equal. In advanced forms, such as the teleost, the 

 base of the fin may be scaled and scales may even cover all 

 or most of the fin. Where such scales e.xist they have no re- 

 lationship with the underlying lepidotrichium. 



The lepidotrichia o{ Polyptems are of interest, because they 

 have denticles and an ornamentation of ridges of dentine 

 and enamel. Separate tooth-bearing plates may occur on the 

 underside of the pectoral fins between the fin rays. The 

 spines of the dorsal finlets of Polypterus are formed from 

 modified lepidotrichia as are the spines of teleosts. 



The margins of the fins of teleosts and the adipose fin, 

 where such occurs, are supported by unjointed horny rods 

 which resemble ceratotrichia. These rods have been called 

 actinotrichia because of their finer structure. In the grow- 

 ing fin, one observes the margin supported by these rods 

 which are replaced by the more superficial lepidotrichia 

 with further growth. 



Embr/o/ogjcal development Ontogenetically, the various 

 types of fin rays arise in the same way. They appear 

 as thickenings of the basement membrane of the epidermis 

 which are cut away by the movement of mesenchyme cells 

 between this thickening and the membrane (Figures 8-47, 

 8-48). They tend to sink from here into the dermis. With 

 the first generation of rays sinking into the dermis, a sec- 

 ond may form, as is seen in the shark. 



This condition demonstrates that actinotrichia or cerato- 

 trichia, which shall be considered as homologs, represent 

 only one expression of a dermal support response which also 

 produces the lepidotrichia. Whether these represent genera- 

 tions of scales, as has been suggested by Jarvik, is another 

 thing, but certainly they involve some of the factors and 

 interactions which produce scales — this is evident in the case 

 of the lepidotrichia. 



General observof/ons The relationship between fin rays 

 and the agnath type of "radials" is not understood but the 

 two never occur together. This suggests that they may rep- 

 resent essentially the same thing. Ceratotrichia fused at the 

 midline could form the agnath radial; stated another way, 

 the agnath radial split into bilateral components could give 

 rise to the ceratotrichia. This ceratotrichial-radial relation- 

 ship is suggested in Bothnolepis, and the jointed radials of 

 the ray may be only incipient ceratotrichia. 



Only in the teleost fin are there two clearly differentiated 

 generations of elements: the actinotrichia supporting the 

 margin and the lepidotrichia the rest of the fin. The inter- 

 relationships of these might suggest the view that there is an 

 outer and inner component — only the inner appears in the 

 shark (but several generations of these!), while both are 

 present and joined in Neoceratodus. The grooved inner sur- 

 faces of lepidotrichia in general suggests the presence of a 

 "ghost" part. 



A simple explanation of all the observed facts does not 

 appear to be possible, for we seem to be dealing with 

 a response system capable of many products. 



TEETH 



Teeth are presumed to be homologs of scales and are so 

 considered here. The teeth cover the margins of the jaws, 



242 • THE SKIN AND ITS DERIVATIVES 



