198 fHE SEA SHORE 



the water immediately enters a large cavity between the mantle 

 lobes. This cavity (the branchial cavity) contains gills, as we 

 have already seen, and also extends to the mouth. The water, 

 urged on by the motion of myriads of minute ciliated cells in the 

 walls of the cavity, passes in part through the digestive tube, and 

 in part around, between, and through the gill plates, which are per- 

 forated by numerous holes. After thus completely bathing the gills, 

 and supplying the oxygen necessary for respiration, this latter 

 current passes into a second cavity above the gills, and thence into 

 the exhalent siphon, where it mingles with the fluid from the diges- 

 tive tube as well as with other excretory matter. 



Lamellibranchs are, as a rule, exceedingly prolific, a single indi- 

 vidual of some species discharging more than a million ova in one 

 season. The larvae swim freely in the water, and are provided with 

 eyes that enable them to search for their food, but the eyes always 

 disappear when the young settle down to a more sedentary life. It 

 is true that adult bivalves sometimes possess visual organs, often in 

 the form of conspicuous coloured spots on the edge of the mantle, 

 these, however, are not the same that existed during the larval 

 stage, but are of a more recent development. 



Lamellibranchs are classified in various ways by different 

 authorities, the arrangement being based principally on the number 

 and position of the adductor muscles, or on the nature of the gills. 

 For our present purpose we shall look upon them as consisting of 

 two main divisions the Asiphonida and the Siphonida, the former 

 including those species which do not possess true tubular siphons, 

 the inhalent and exhalent openings being formed merely by the 

 touching of the mantle lobes ; and the latter those in which the 

 mantle lobes are more or less united and tubular siphons formed. 

 Each of these divisions contains a number of families, most of 

 which have representatives that inhabit the sea ; and we shall now 

 note the principal characteristics by which the more important 

 families are distinguished, and take a few examples of each, starting 

 with the Siphonida. 



Examining the rocks that are left exposed at low tide we fre- 

 quently find them drilled with holes that run vertically from the 

 surface, seldom communicating with each other within, and vary- 

 ing in diameter from less than a quarter of an inch to half an inch 

 or more. Some of these holes are the empty burrows of a boring 

 mollusc, while others still contain the living animal in situ. 



The molluscs in question belong to the family Pholadidce, 



