V.] THE FRESH-WATER MUSSEL. 309 



one communicating with the surrounding medium through 

 the agency of the siphonal prolongations of the mantle 

 border alone. The walls of the two chambers and the 

 surfaces of all the organs contained within them, together 

 with the whole lining membrane of the alimentary canal, 

 are ciliated, and the currents thereby induced set in, as 

 before stated, by the lower and out by the upper siphon. 

 One of three courses is open to the inhalent current with its 

 suspended food material it may pass either through the 

 gills, through an interspace between the gills and the body 

 wall, or into the mouth. Take whichever course it may, it 

 finally reaches the supra-branchial chamber. It follows that 

 the insetting current is both a nutritive and a respiratory 

 one, while the outsetting one serves to carry away the 

 waste products of respiration and digestion, together with 

 the products of the excretory and genital organs. 



The clue to the real meaning of the more important 

 structural features in the organization of this animal, is to 

 be sought in an understanding of the above facts. 



Digestion, that is solution of the proteinaceous and other 

 nutritive matters contained in food, is effected in the sto- 

 mach and intestine; and the nutritious fluid, thus formed, 

 transudes through the walls of the alimentary cavity and 

 passes into the blood contained in the blood-vessels which 

 surround it. This blood is thence carried into a large sinus, 

 which occupies the middle line of the body under the peri- 

 cardium and between the organs of Bojanns (see Laboratory 

 Work E), and receives the greater part of the blood return- 

 ing from all parts of the body. From this median vena cava, 

 branches are given off to the gills and open into .the exten- 

 sive vascular network which those organs contain. From 

 this, again, trunks lead towards the pericardium and open 

 into one or other of the two auricles of the heart, which 



