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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 : — 1st, a series of inter- 

 filamentar junctions {Br.j.l, figs. 26 and 28, PI. V.) joining 

 the separate filaments in each lamella, and 2nd, a series of 

 inter-lamellar junctions {Br.j.2, fig. 26) joining the two 

 lamellae of the same branchia. It will appear from a 

 consideration of figs. 26 and 28 that this conjunctive 

 tissue is 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 

 intor-filamcntar junctions. But there must also be a 



