xii PHYLUM MOLLUSCA 671 



bounded above aud 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.), aud are covered with ciliated epithelium, the 

 large cilia (Fig. 577, D) 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. The outer lamella of the outer lamina is attached 

 along its whole length to the mantle (Fig. 578) : the inner lamella 

 of the outer and the outer lamella of the inner lamina are attached 

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

 of the mantle : the inner lamella of the inner lamina is also attached 

 to the visceral mass in front (A), but is free further back (B). The 

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

 the posterior adductor (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 arrange- 

 ment it will be seen that the water-tubes all open dorsally into a 

 supra-branchial chamber (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 produced which sets in 

 through the inhalant siphon into the pallial cavity, through the 

 ostia into the water-tubes, into the supra-branchial chamber, and 

 out at the exhalant siphon. The in-going current carries with it 

 not only oxygen for the aeration of the blood, but also Diatoms, 

 Infusoria and other microscopic organisms, which are swept into 

 the mouth by the cilia covering the labial palps. The out-going 

 current carries with it the various products of excretion and the 

 faeces passed into the cloaca. The action of the gills in producing 

 the food-current is of more importance than their respiratory 

 function, which they share with the mantle. 



The excretory organs are the kidneys or urocaeles (portions 

 of the true coelome), situated one on each side of the body just 

 below the pericardium. Each consists of two parts, a brown 

 spongy glandular portion or kidney (Fig. 576, kd.), and a thin- 

 walled non-glandular part or urinary bladder (bl.), which com- 

 municates with its fellow anteriorly by a large oval aperture (x). 

 The two parts lie parallel to one another, the bladder being placed 

 dorsally and immediately below the floor of the pericardium : 

 they communicate with one another posteriorly, while in front each 

 glandular part opens into the pericardium (>. />. ap.), and the bladder 



