SEA-FISHERIES LABORATORY. 2938 
double series of hollow filaments which were primitively 
vascular channels, the wall of the filament itself serving 
as the membrane through which the gaseous interchange 
between the blood and the surrounding water is effected. 
This simple arrangement is complicated here by the 
process of folding, which is described above, and further 
by a partial coherence of the filaments, which with the 
development of other vascular tissues, form two series of 
junctions within the branchia:—Ist, a series of inter- 
filamentar junctions (677.1, figs. 26 and 28, Pl. V.) joming 
the separate filaments in each lamella, and 2nd, a series of 
inter-lamellar junctions (Brj.2, fig. 26) joining the two 
lamelle of the same branchia. It will appear from a 
consideration of figs. 26 and 28 that this conjunctive 
tissue 1s not formed simply by the branchial filaments 
themselves, but also by vascular tissue developed from 
the base of the ctenidium. The whole of an inter-lamellar 
junction is constituted by this vascular tissue, the inter- 
filamentar junction, on the other hand, is formed both 
by the union of the adjacent walls of the filaments and 
by a separate vascular tissue. Wherever such an inter- 
filamentar junction occurs, the filaments taking part in 
it have split (fig. 28) and the adjacent edges of separate 
filaments have united. But underneath this place of 
union the vascular channel is completed by a sheet of . 
connective tissue continuous with the tissue of the efferent 
or afferent vessels, as the case may be. If the whole 
lamella could be flattened out, it would appear as a 
trellis work of which the vertical bars would be formed by 
the filaments, the horizontal bars by the vascular inter-fila- 
mentar junctions. At intervals of every 40 or 50 filaments, 
vertical afferent and efferent vessels occur alternately, and 
between these vessels the blood circulates in the horizontal 
inter-filamentar junctions. _ But there must also be a 
