272 MANUAL OF ZOOLOGY SECT. 



surface each gill presents a delicate double striation, being 

 marked by faint lines running parallel with, and by more 

 pronounced lines running at right angles to, the long axis of 

 the organ. Moreover, each gill is double, being formed of 

 two similar plates, the inner and outer lame lice, uniting with 

 one another along the interior,. 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 (Figs. 162 and 

 163) ; its cavity is subdivided by vertical bars of tissue, the 

 inter-lamellar junctions (2. /./.), which extend between the 

 two lamellae, and divide the intervening space into distinct 

 compartments or water tubes (w. t) 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 fila- 

 ments are connected by horizontal bars, the inter-filamental 

 junctions (/././). 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-filamental 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 



