272 MANUAL OF ZOOLOGY SECT. 



one another along the anterior, ventral, and posterior edges 

 of the gill, but free dorsally. The gill has thus the form of 

 a long and extremely narrow bag open above : its cavity is 

 subdivided by vertical bars of tissue, the inter-lamellar 

 junctions (Fig. 151, /././.), which extend between the two 

 lamellae and divide the intervening space into distinct 

 compartments or water tubes (./.), closed ventrally, but 

 freely open along the dorsal edge of the gill. The vertical 

 striation of the gill is due to the fact that each lamella is 

 made up of a number of close-set gill-filaments (/), the 

 longitudinal striation to the circumstance that these 

 filaments are connected by horizontal bars, the inter-fila- 

 mentar junctions (t.fj.). At the thin free or ventral edge of 

 the gill the filaments of the two lamellae are continuous 

 with one another, so that each gill has actually a single set 

 of V-shaped filaments, the outer limbs of which go to form 

 the outer lamella, their inner limbs the inner lamella. 

 Between the filaments, and bounded above and below by the 

 inter-filamentar junctions, are minute apertures, or ostia (os.\ 

 which lead from the mantle-cavity through a more or less 

 irregular series of cavities into the interior of the water-tubes. 

 The filaments themselves are supported by chitinous rods 

 (r.\ and are covered with ciliated epithelium, the large 

 cilia of which produce a current running from the exterior 

 through the ostia into the water-tubes, and finally escaping by 

 the wide dorsal apertures of the latter. The whole organ is 

 traversed by blood-vessels. 



Owing to this arrangement it will be seen that the water- 

 tubes all open dorsally into a supra-branchial chamber, 

 continuous posteriorly with the cloaca, and thus opening 

 on the exterior by the exhalant siphon. 



The physiological importance of the gills will now be 

 obvious. By the action of their cilia a current is produced 



