XI MOLLUSC A 145 



lies in the cavity just above the gills ; it can often be seen 



without further dissection through the transparent walls of 



this cavity. Thence the blood passes into the single ventricle 



and is distributed again to all parts of the body. The fluid is 



colourless, and hence the path of the circulation cannot be 



demonstrated without special injection and dissection of the 



vessels. 



„ . ... The same water current that carries to the aills 



Nutrition. ,, , . ,. . °, 



the oxygen necessary for respiration, carries also 



the microscopic organisms which serve the mussel for food. 



The mcmth of the mussel lies in front just above the foot, 

 and is best found by probing with a " seeker." On either 

 side of the mouth lie two triangular palps having much 

 the same appearance and texture as the gills ; the outer 

 palps unite over the mouth, forming the upper lip ; the inner 

 palps unite below the mouth, forming the lower lip. These 

 palps, like the gills, are covered with lashing hairs or cilia. 

 When the in-current of water passes through the gills, it 

 leaves behind it all the organisms and solid matter suspended 

 in it, which are too coarse to pass through the gill structure ; 

 thus a slimy rope of organic matter gradually accumulates 

 in the groove between the two pairs of gills. This organic 

 substance is passed forward between the palps by the action 

 of the cilia, and onward into the mouth, whence it is driven in 

 a similar way into the stomach. There it is acted upon by 

 the digestive juices of the large, so-called "liver gland," 

 and passed on into the intestine, the inner walls of which 

 are lined with cilia. The intestine runs down from the 

 stomach into the upper part of the foot, where it follows a 

 curiously irregular course, coiling backwards and forwards, 

 then turning upward and running right through the heart, 

 and ending close behind the exhalent aperture. Any excreta 

 are carried out with the exhalent water current. 



The mussel seems to have, to some extent, the power of 

 controlling the nature of the food substances which are allowed 

 to pass in with the inhalent current ; this is due doubtless 

 to the sensitive tentacles which surround the inhalent aperture. 

 When the approach of any undesirable substance is detected 

 by these tentacles, a sudden exhalent discharge of water 

 takes place from hoih apertures, driving the harmful matter 

 away. 



VOL. I L 



