xxviii EXCRETORY ORGANS 359 



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 (Fig. 94), as far as the posterior 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 supra-branchial chamber, con- 

 tinuous 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, thence 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 a single pair of curiously-modified 

 fiephridia, situated one on each side of the body just below 

 the pericardium. Each nephridium consists of two parts, a 

 brown spongy glandular portion or kidney (Fig. 94, kd), and 

 a thin-walled non-glandular part or bladder (bl). The two 



