THE DERMAL FIN-RAYS OF FISHES. 479 



which pass inwards below the body-scales, as in the caudal 

 fin, and are not related to a particular baseost element, as in 

 the higher fish described above. The lepidotrichia of the 

 ventral caudal lobe bear the same relation to the endo- 

 skeleton. In their histological structure the dermal rays 

 closely resemble those of Lepidosteus. 



On examining sections through the growing edge of the 

 fins, we find very well developed actinotrichia (fig. 17). The 

 lepidotrichia arise, as usual, as a thickening of the basement 

 membrane. As they grow older the segments gradually 

 acquire a structure exactly similar to that of the body-scales, 

 with a covering coat of gauoine (figs. GO and 61). Moreover 

 the young rays are provided on their outer surface with a 

 number of small denticles (figs. 18 and 60) like those found 

 in Lepidosteus and Hypostoma. There can bo no possible 

 doubt that the actinotrichia and lepidotrichia of Polypterus 

 are strictly homologous with the actinotrichia and lepido- 

 trichia of the '' Actinoptcrygii " (Teleostei and Ganoidei). 



Acipenser. 



Hertwig(17) has already described the dermal rays of the 

 Acipenseroids. In adult sturgeons the actinotrichia are very 

 well developed at the edge of the fins. The lepidotrichia, 

 originating just as in the Teleostei and " bony Ganoids,^' 

 soon acquire a bony structure like that of the body-plates 

 and scales, and also small bony spines, wliich are considered 

 by Hertwig to represent degenerate denticles. 



In the Acipenseroids, or Ohondrostei, the lepidotrichia are 

 much more numerous than the endo-skeletal fin-rays. Their 

 proximal ends overlap the cartilages, especially in the 

 impaired fins, more than is the case in the higher fish ; and 

 they are not so closely connected with the endo-skeleton. In 

 fact, the general relations of the lepidotrichia to the endo- 

 skeleton and muscles somewhat resemble those of the cerato- 

 trichia in Elasmobranchs. They are shown in fig. 2, and 

 Text-fig. 3, p. 487. 



