CLASS LAMELLIBRAXCHIATA. 



6i 



lives. Tlie animal lives buried in the sand, and buried 

 head downwards, so to speak — that is to say, the mouth 

 is turned downwards, and the end where the " siphons " 

 or breathing-tubes are, is turned upwards (as in fig. 24). 

 The breathing-tubes are two long muscular canals, which 

 are so united to one another as to look like one tube, though 

 really quite distinct internally. They can be thrust out 

 of the shell at will, and again partially withdrawn within 

 the shell by means of proper muscles. When the animal 

 wishes to breathe or obtain food-, it thr^ists out these 

 breathing-tubes through the sand in which it is buried 

 till they reach the water above. Then the water is drawn 

 in through the mouth of one of the tubes in a constant 

 current and is carried to the gills in the interior of the 

 body (see figs. 24 and 25, where the direction of the 

 water-currents is indicatetl by arrows). Having passed 

 over the gills, and purified the blood in its passage, the 



pi 



Fig. 26.— Interior of tiie left valve of Mija armaria, b Beak ; a Front end 

 of the sliell ; p Hinder end of the shell ; ad Soar or impression of the front 

 adductor muscle ; JJ(7 Impression of the hinder adductor ; JJS Line where 

 the muscles which pull the siphon in are attached : jil Line where the mantle 

 is attached ; s Spoon-ahaped process carrying the " cartilage." 



water next reaches the mouth, and the animal extracts 

 from it all the floating particles of food which it may 

 contain. Finally, the water is conducted in a reverse 



