VII GILLS 389 



or ventral edge of the gill the filaments of the two lamella; 

 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, and 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 ((^. v). 



The mode of attachment of the gills presents certain 

 features of importance (compare Fig. 95, A, B, C). The 

 outer lamella of the outer gill is attached along its whole 

 length to the mantle : the inner lamella of the outer, and 

 the outer lamella of the inner gill are attached together to 

 the sides of the visceral mass a little below the origin of the 

 mantle : the inner lamella of the inner gill is also attached to 

 the visceral mass in front, but is free further back. The gills 

 are longer than the visceral mass, and project behind it, below 

 the posterior adductor (Figs. 93 and 95 C), as far as the pos- 

 terior edge of the mantle : in this region the inner lamellae of 

 the inner gills are united with one another, and the dorsal 

 edges of all four gills constitute a horizontal partition between 

 the pallial cavity below and the exhalant chamber or cloaca 

 above. Owing to this arrangement it will be seen that the 

 water-tubes all open dorsally into a stipra-braiichial chamber 

 (Fig. 95, s. br. c), continuous posteriorly with the cloaca and 

 thus opening on the exterior by the exhalant siphon. 



The physiological importance of the gills will now be 



