THE DKRMAL FIN-KAYS OF FISHES. 483 



they are present in the quite young stages of development ; 

 but they are not figured by Semon (33). 



The Structure and Homologies of the Dermal Eays of 



Living Fish. 



We may now briefly recapitulate what lias been said above 

 concerning the dermal rays of liviug fish, and discuss their 

 homologies. 



There are four different kinds of dermal rays, or dermo- 

 trichia, in adult living fish : 



i. The horny rajs, or ceratotrichia, of Elasmobrauchii 

 and Holocephali : fibrous, flexible, un jointed, rarely branched, 

 cylindrical rods. Like all dermal raj^s, they are developed on 

 both sides of the fin-folds both of the median and of the 

 paired fins. The pointed proximal ends overlap on each side 

 the median endo-skeletal fin-supports, and the tapering grow- 

 ing distal ends reach the free edge of the fin. Ceratotrichia 

 are of the same structure throughout, grow by the addition of 

 concentric layers of substance, and never contain any bone- 

 cells. They originate as fibrils secreted by mesoblastic sclero- 

 blasts in contact with the inner surface of the basement 

 membrane. As they grow older they sink inwards, new layers 

 of connective tissue being formed outside them. During the 

 growth of the fin new generations of ceratotrichia may be 

 added, so that a section of a fully grown fish may show a 

 layer of rays several deep on either side. The bony denticles 

 of the skin lie outside, and are quite independent of the 

 underlying rays, which are the first to make their appearance 

 in the embryo (Text-fig. 1). 



2. The actinotrichia : delicate little "horny/' un- 

 jointed, and usually unbranched rays found in the fins of all 

 living Teleostomes as the first, and for some time the only, 

 rays in embryonic or larval life. In later stages they become 

 functionally replaced by the bony rays, except in the adipose 

 fin. The actinotrichia, however, persist throughout life near 

 the free growing edge of all the fins. They develop like the 

 ceratotrichia, and appear to be in every respect similar to 



