404 THE MUSSEL CHAP. 



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 junc- 

 tions, 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 (b. v). 



The mode of attachment of the gills presents certain 

 features of importance (compare Fig. 103, A, B, C). The 

 outer lamella of the outer lamina is attached along its 

 whole length to the mantle : the inner lamella of the 

 outer and the outer lamella of the inner lamina are 

 attached together to the sides of the visceral mass 

 little below the origin of the mantle : the inner lamella 

 of the inner lamina 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. 101 and 103, C), as far as the 

 posterior edge of the mantle : in this region the inner 

 lamellae of the right and left inner laminae are united with 

 one another, and the dorsal edges of all four laminae 

 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 supra-branchial chamber 

 (Fig. 103, 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 

 obvious. By the action of their cilia a current is pro- 

 duced which sets in through the inhalant siphon into 



