HABITS AND MOVEMENTS OF THE RAZOR-SHELL CLAM. 13! 



shell is closed. The reason for this arrangement is found by 

 studying the movements of burrowing and swimming. 



The siphons have nearly circular or slightly elliptical openings, 

 and are separated near their extremities. The whole posterior 

 end of the mantle, bearing the siphons, may be protruded some 

 distance beyond the posterior end of the shell. Sense tentacles 

 surround the siphons near their bases and occur on the mantle 

 dorsally and ventrally near the posterior end, where the mantle 

 is exposed between the shell valves (Figs. 2 and 5). Small 

 sense tentacles occur on the surfaces of both siphons, and the 

 branchial siphon bears a number along its extremity that tend to 

 radiate in over the opening of this siphon. The margin of the 

 cloacal siphon has no tentacles. Verrill and Smith have (7) de- 

 scribed a definite arrangement for the tentacles but it is doubtful 

 if this arrangement always holds. 



The ventral margins of the mantle lobes are united throughout 

 their length except near the middle of the length of the animal, 

 where a small opening remains that is situated just posterior to 

 the retracted foot (Fig. 4). This opening is surrounded by a 

 single row of sense tentacles. Except for this opening, the 

 opening through which the foot is protruded, and the openings 

 of the siphons, the mantle forms a closed chamber. 



The united mantle margins are very muscular, being provided 

 with strong circular and radial pallial muscles that are very simi- 

 lar to the muscles of the mantle margins in Solenoinya (2) where 

 they serve very much the same purpose, that is, to close the 

 shell tightly and to obliterate a portion of the mantle chamber. 

 Like Solenoinya the valves of the shell are covered with a very 

 heavy, elastic cuticle that is extended beyond the calcareous mar- 

 gins. Mud does not readily adhere to this cuticle. When the 

 valves are closed the cuticle is bent in over the hard margins of 

 the shell (Fig. 8), thus allowing the united margins of the mantle 

 to be withdrawn. Probably this elastic cuticle aids in opening 

 the mantle chamber when the muscles relax, as is undoubtedly 

 the case with Solenoinya (2), but the effect in this form is cer- 

 tainly much less than in Solenoinya. 



Both adductor muscles are present. The anterior adductor is 

 very large and strong. The posterior adductor is quite small 

 and does not seem to function actively. The united margins of 



