88 



Each, branchial arch (Text-fig. 4, A.) consists of several 

 pieces, and is for the most part a densely calcified tube, 

 the ends of which are cartilaginous. The interior cf the 

 tube is strengthened by bony trabeculae. The gill fila- 

 ments are borne on the posterior and convex borders of 

 the gill arches. On first inspection it may appear that 

 there is only one series, but closer study shews that there 

 are really two. This is particularly noticeable in the 

 anterior gills, where the two series of filaments are of 

 unequal length, so that all the anterior (or external) are 

 longer than the posterior or internal ones. The bases of 

 all the filaments borne on one branchial arch are fused 

 together, but the greater portions of them are free from 

 each other. In section (Text-fig. 4, B.) each filament is an 

 isosceles triangle. They are so disposed that the apices 

 of the triangles are directed towards each other and those 

 of the one series alternate with those of the other. 

 Obviously this arrangement secures the greatest economy 

 of space consistent with the size of the filaments. 



Text-figs. 4 are a diagrammatic representation of 

 the structure of the gill filaments. Fig. A. is a 

 diagrammatic transverse section of a gill arch, and shews 

 two filaments belonging to adjacent demibranchs. Each 

 filament is supported by a cartilaginous rod — the gill ray 

 which runs down in its axis. These gill rays are super- 

 ficially calcified ; their proximal ends are swollen and are 

 all fused together, but the connecting portions are not 

 calcified. The skeleton of a demibranch is therefore a 

 comb-like structure. The rays of the adjacent demi- 

 branchs are placed alternately, so that the knob-like calci- 

 fied proximal end of one ray is placed opposite the car- 

 tilaginous connecting portion of the two opposed ones. 

 Dense ligamentous bands connect the fused heads of the 

 gill rays with the branchial arch and with each other. 



