THE LAMELLIBRANCHIA 209 



exhalant in this genus ; but is posterior and only exhalant in Lutraria, 

 T/iracia, etc. 



Frequently the two posterior pallial orifices, anal and branchial, 

 or at least the anal, as in some Lucinidae, are more or less pro- 

 longed in the form of muscular tubes which may be extended for a 

 greater or less distance beyond the shell (Fig. 190) ; this feature is 

 found in the majority of burrowing and boring Lamellibranchs. 

 These tubes are known as siphons, and may be either (1) inde- 

 pendent of one another throughout their length, as, for example, in 

 the Tellinidae (Figs. 190, Ir.s, a.s ; 245, g, g), Donacidae, Thracia, 

 etc. ; or (2) partially fused together, as may be seen in Tapes, 

 Solenocurtus (Fig. 194), Saxicava (Fig. 246); or (3) completely 

 united to one another, as in Mactra, Dosinia, Mya. Liitntria, Pholas, 

 Teredo (Fig. 195), etc. The branchial siphon is usually the longer 

 of the two, and in Scrobiculuria is more than four times as long as 

 the body. Sometimes the siphons may attain to a still greater 



Fio. 190. 



Tellina planata, left-si<le view, a..-?, anal siphon ; ?<r..<, branchial siphon ; /, foot ; pa, 

 tentaculiferous border of the mantle ; sh, shell. (After Poli.) 



degree of development, and may form an important part of the 

 body-mass, or even surpass it in size. The extreme limit of evolu- 

 tion in this direction is exhibited by Teredo, in which these organs, 

 while receiving the same nerve supply as in other Lamellibranchia, 

 constitute the greater part of the mass of the animal ; the anal 

 siphon extends right forward and the branchial siphon contains the 

 gills, as is also the case in Solenocurtus, Fistulana, Saxicava, etc. 



There are two opposite currents in the pallial cavity. The first 

 is postero-anterior in direction, and is set up by the action of the 

 cilia clothing the surface of the branchiae ; the second runs in the 

 reverse direction on the ventral side near the edges of the mantle, 

 and wards off foreign substances from the pallial cavity (Mytilus, 

 Cardium, etc.). In Pinna and Solen this second current is created 

 by a large longitudinal ciliated ridge on each side. 



The two lobes of the mantle are furnished with several distinct 

 pallial muscles which are inserted on the shell and are divisible into 

 the following distinct groups : (1) The orbicular muscle, extending 

 right round the periphery of each lobe, and serving to attach it to 



14 



