MOLLUSCA 65 



protruding from it ventrally are the median foot, and laterally 

 the two pairs of gills and the folds around the mouth known as 

 labial palps. The space bounded by the mantle folds and con- 

 taining these organs is the mantle or pallial cavity. The gills 

 are large expansions of the body wall on each side and are 

 developed as niters of the water introduced into the mantle 

 cavity through the inhalent opening. They are disposed in 

 flattened outer and inner pairs, and each consists of an outer 

 and an inner lamella enclosing a cavity. Each lamella is 

 pierced by vertical and horizontal slits dividing the lamella 

 into filaments, and these put the mantle cavity in communica- 

 tion with the inner cavity of the gills. The gill filaments are 

 strengthened by chitinous rods and are connected by inter- 

 filamentar junctions at intervals, and the lamellae are similarly 

 connected by interlamellar junctions. The outer lamella of 

 the external pair of gills is continuous with the inner face of 

 the mantle, the inner lamella with the outer lamella of the 

 internal gill. The inner lamella of the latter is attached 

 anteriorly to the ectoderm of the visceral mass. It ends 

 freely, however, on either side of the foot, and behind the foot 

 the inner lamellae of the two inner gills fuse, and by further 

 fusion the cavities of the gills are reduced to a single cavity 

 which discharges through the exhalent opening, a cavity which, 

 since it receives besides the excretory products from the 

 kidneys and the egesta from the alimentary canal, has been 

 called a cloaca. 



INTERNAL MORPHOLOGY. Alimentary Canal. The mouth 

 lies medianly between the anterior adductor muscle and the 

 foot, and it is fringed with lips which are drawn out on each 

 side into ciliated sensory processes, the labial palps. These 

 take up the food gathered by the mantle cavity. The mouth 

 leads upwards by a short oesophagus to the stomach, a wider 

 part of the tube with folded walls which receives the openings 

 of the digestive gland. The tubules of the gland form a dark 

 mass around the stomach, and the organ is usually known as 

 the liver. The intestine leaves the stomach on the ventral 

 side and passes into the foot, and after a course in the foot, 

 indicated in fig. 30, it is directed dorsally and then horizon- 

 tally to pass through the pericardial cavity. In this cavity 



