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 of the 
tube is strengthened by bony trabecule. 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. Hach 
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 calei- 
fied proximal end of one ray is placed opposite the car- 
tilaginous connecting portion of the iwo opposed ones. 
Dense ligamentous bands connect the fused heads of the 
gill rays with the branchial arch and with each other. 
