304 MOLLUSCA [CH. 



the small holes left between the gill-filaments and so into the 

 epibranchial chamber and escapes by the dorsal siphon, carrying 

 with it the matter cast out from the kidneys and the anus. As this 

 water percolates through the gills the blood which circulates in 

 these organs receives oxygen from it and gets rid of carbonic acid. 



In front of the gills there are situated two organs called labial 

 palps, on each side of the anterior part of the animal (8, Fig. 13*2). 

 These are triangular flaps, an upper and lower on each side, the 

 surfaces of which are covered with grooves clothed with abundant 

 cilia on the sides turned towards one another. The two superior 

 labial palps are connected by a narrow ridge crossing above the 

 mouth ; the two inferior labial palps by a similar ridge beneath it. 

 The mouth thus lies at the bottom of a trough, the lips of which 

 are formed by the superior and inferior labial palps respectively. 

 The mouth is situated beneath the great anterior adductor muscle 

 which projects beyond it like a forehead. Not all the cilia on the 

 grooves crossing the labial palps however serve to convey food 

 to the mouth; some on the contrary serve to remove excess of 

 food from the mouth. It has been shown that on each ridge 

 separating two grooves the cilia on the posterior surface beat 

 towards the mouth but those on the opposite side away from the 

 mouth. As in the natural position the ridges tend to incline and 

 overlap each other towards the mouth the cilia which are effective 

 are those which drive food to the mouth. When the animal has had 

 enough or the food is distasteful a nervous impulse causes blood to 

 be forced into the ridges ; they are erected and then the repulsive 

 cilia remove the unused food from the mouth. This food would 

 decay and form a breeding ground for harmful germs if it were not 

 got rid of. Food is conveyed to the mouth by a band of short cilia 

 borne by the strip of ectoderm running along the lower edge of the 

 inner gill-plate where the descending and ascending limbs of the 

 filaments pass into each other. The so-called " frontal cilia" which 

 clothe the outer sides of the ascending arid descending limbs of the 

 filaments of the inner gill-plate produce currents which flow down- 

 wards towards this mid ventral strip of ectoderm and wash towards 

 it all particles which are too large to pass through the gill-pores. 

 The ventral strip of ectoderm contains not only ciliated cells but 

 also glandular cells which produce mucus. In this secretion the minute 

 organisms contained in the water are ensnared and the whole cord of 

 mucus thus produced is worked forwards towards the mouth at the 

 anterior edge of the gill-plate where it falls on to the palps and is by 

 them taken to the mouth. The rejected food is transferred from the 



