LAMELLIBRANCHIATA. 69 1 



are either no siphons or but short ones. On the other hand, in 

 those Bivalves which have long respiratory siphons there exists on 

 each side a specially developed " retractor muscle," the function of 

 which is to withdraw the siphons within the shell. The insertion of 

 this siphonal retractor causes an indentation in the pallial line pos- 

 teriorly (fig. 556, b, s), the depth of this depending upon the size 

 of the siphonal muscles. In all those Bivalves, therefore, which 

 possess retractile siphons, the pallial line is deflected posteriorly into 

 a larger or smaller " pallial sinus " or " siphonal impression ; " and 

 those Bivalves in which this sinus exists are grouped together under 

 the name of Sinupattialia. 



There is no distinctly differentiated head in any of the Lamelli- 

 branchs (hence the name of "Acephalous Molluscs" commonly 

 given to the class), and the mouth is simply placed at the anterior 

 end of the body. It is furnished with ciliated, leaf-like, membranous 

 processes, or "labial palpi" (fig. 555, /), which are two or four in 

 number, and serve to conduct the in-going water-currents to the 

 mouth. The mouth is not furnished with any arrangement of teeth, 

 and the animal lives upon the microscopic particles of nutrient 

 matter brought to the mouth by the in-going currents of water, 

 which are kept in movement by the action of the cilia covering the 

 gills. These organs (fig. 555, g) are leaf-like, and are attached by 

 their bases to the sides of the body, their free ends depending into 

 the mantle-cavity. Most Bivalves have two of these lamellar 

 branchiae on each side of the body, but the external pair may be 

 wanting. 



As has been already mentioned, the arrangements for the admis- 

 sion of water to the gills, and its expulsion again from the mantle- 

 cavity, are essentially the same in all Bivalves. In almost all cases 

 the in-going current of water enters the pallial chamber posteriorly 

 and ventrally, while the out-going current escapes posteriorly and dor- 

 sally. In those Bivalves which have free mantle-lobes (" Asiphonate 

 Bivalves "), the apertures for the water-currents are simply produced 

 by the apposition of the hinder edges of the mantle-lobes to each 

 other. On the other hand, in those Bivalves which have the mantle- 

 lobes united ("Siphonate Bivalves," fig. 555), the margins of the 

 " inhalant " and " exhalant " apertures are drawn out or extended 

 into longer or shorter muscular tubes or " siphons." The siphons 

 may be separate, or they may be united to one another along one 

 side, and they can usually be partially or entirely retracted within 

 the shell by means of special muscles, called the " retractor muscles 

 of the siphons." These siphons are more specially characteristic of 

 those Lamellibranchs which spend their existence buried in the sand, 

 protruding their respiratory tubes in order to obtain water, and with 

 it such nutrient particles as the water may contain. As has been 



