VARIETIES OF OYSTERS. 135 



This last arrangement is carried to an extreme in those 

 species which burrow and live in holes of the rock or mud 

 of the sea-bottom. The only communication which these 

 have with the outer world is by means of their extended 

 tubes or siphons, as they are called. In their case the two 

 tubes are united into one sheath, although a partition 

 passing down the double tube always keeps them function- 

 ally distinct. In these creatures the action of the cilia 

 drives the water in one continuous stream from the gill 

 chamber to the aerial chamber through the tubes ; and this 

 motion necessitates a flow down one tube and up another. 

 By this means floating food is passed along the gills to 

 the mouth, which is situated at the lower end of the buried 

 molluscs, and thus we have a similar mode of life to that 

 of the tunicates, though the breathing apparatus is of a 

 different structure. 



The mantle being thus relieved from the duty of 

 exposing the blood to the oxygen dissolved in the water, 

 is wholly employed in its proper function of secreting the 

 shell. It performs this office in a very efficient manner, so 

 as always to allow for the growth of the animal and for the 

 strengthening of the shell as the contained animal becomes 

 more weighty, and therefore liable to experience more 

 violent collisions. 



The method of secretion is the following : Round 

 the edge of the mantle lobes, or at that part where they 

 leave the shells, are situated a great number of glands, 

 whence secretions of different substances are poured out 

 and mingled together. These glands secrete horny mat- 

 ter, a large quantity of carbonate of lime, and some 

 pigment. Thus a fresh rim of hard matter is added at 

 intervals to the shell. The size, shape, markings, and 



