218 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



quite firm, somewhat triangular in shape, and is capable of 

 being extended beyond the edges of the shell for a considerable 

 distance. It is by means of this organ that the animal plows 

 its way through the mud or sand in which it lives. The gills 

 are two pairs of flattened, ribbed, membranous folds which hang 

 down into the mantle cavity from each side of the body. The 

 water bathing these gills and passing up between them comes 

 in very close contact with the minute blood-vessels with which 

 the gills are abundantly supplied so that the transfer of gases 

 from the water to the blood and from the blood to the water 

 can readily take place. The body is not divided into well- 

 defined regions. Just below and back of the anterior adductor 

 muscle is the mouth opening. On each side of it, looking some- 

 what like little gills, are two pairs of labial palpi whose function 

 it is to convey to the mouth the minute plant or animal organ- 

 isms that are carried in by the water. Through a short esopha- 

 gus these particles, which are the food of the mussel, pass 

 into a rather large stomach and from that into a long narrow 

 intestine which is coiled in the base of the foot and the visceral 

 mass. The posterior part of the alimentary canal, the rectum, 

 is a long straight tube extending through the pericardium and 

 opening into the supra-branchial cavity close to the exhalant 

 siphon. 



The pericardium is the space in the upper portion of the 

 visceral mass just below the hinge-ligament. It is covered by 

 a delicate membrane, and contains the heart and some of the 

 blood-vessels. The heart consists of a single ventricle, which 

 surrounds part of the rectum, and a right and left auricle. 

 When the ventricle contracts it sends the blood forward and 

 backward through large blood-vessels, the anterior aorta and 

 the posterior aorta. Part of the blood is carried directly to the 

 mantle where it is aerated and then returned to the heart. The 

 rest of the blood is carried to various parts of the body and 

 finally collects in a space, the vena cava, just beneath the peri- 

 cardium. From the vena cava the blood passes into the excre- 

 tory organs, the kidneys, which lie just beside it, and on down 

 into the gills and finally back to the heart where it enters the 

 auricles. 



