RESPIRATION. 81 



The lower orifice or siphon is afterent or branchial, the upper 

 one efferent or anal. Movement of the water is produced by 

 vibratile cilia which carpet the branchiae, and which act also in 

 directing the food towards the mouth. In the Siphonida the 

 tubes or siphons are sometimes very long, and either closely- 

 united as in Mya arenaria (iii, 54), or more or less divergent 

 (iii, 53). Most of the free bivalves are siphonated and the length 

 of these organs depends upon the depths to which the various 

 species customarily bury themselves in sand or mud. Mactra 

 soUdissima, a common mollusk of the Atlantic coasts of the 

 United States, is frequently washed shoreward by the waves, 

 which in retiring leave it exposed. Its active foot quickly 

 buries it beneath the surface of the sand, where it awaits the 

 subsequent overflows, its sole communication with the outer world 

 being a hole in the surface leading to the siphonal openings. The 

 pressure of a foot upon the circumjacent wet, yielding sand, causes 

 the ejection of the water contained within the branchial cavity, 

 through the efferent siphon, whence it arises in a forcible stream, 

 several inches, or even a foot or more, into the air. 



Our fresh-water mussels (Unionidse) habitually bury a part of 

 the shell in the river bed, but expose that portion enclosing the 

 siphons; a single species of the Ohio R-iver {Marg. dehiscens, 

 Say), buries itself beneath the surface, communicating with the 

 w'ater through a hole, like the Mactra.* 



IS'on-siphonated bivalves, or those in which the mantle is 

 divided into two lobes, are provided with valvules or folds which 

 render the respiratory irrigation just as complete. 



The respiratory currents have no connection with the opening 

 or closing of the valves of the shell ; these movements being- 

 connected with locomotion, or efforts for the expulsion of irri- 

 tating substances. If an Anodonta be placed in a vessel of 

 water into which some fine sand is introduced, the particles will 

 be seen entering the incurrent siphon and repelled from the 

 orifice of the excurrent one: but after the animal has had enough 

 of the unpalatable and irritating food it will close its valves, 

 forcing out the water, and with it the sand, through both siphons. 



In Ostrea and its related genera the two branchial leaves of 

 each side are equal and exactly superposed, whilst in the 

 Unionidse thej^ are unequal, the interior extending further in 

 front than the exterior one ; in Petricola the external branchiae 

 are prolonged posteriorly bej^ond the line of junction of the four 

 leaves, thus producing a third lateral branchia or a reflected 

 portion of the external branchiae ; in Corbis and Lucina there 

 exists but a single thickened branchia on each side, composed 

 of a number of superposed leaves joined intimately ; in Tellina 



* Lea, Obs., i, 117. 



