1837.] 



ON CYCLESTHERIA HISLOPI. 



33 



continued for any length of time, but take place at intervals, the 

 animal being more freuuently found resting on the bottom ur 

 affixed to some submerged object. 



The creeping or burrowing mode of locomotion, which takes 

 place while the animal is on the bottom, is effected partly by the 

 antennæ, but more especially by the flexion and powerful exten- 

 sion of the trunk, the caudal plate being thus exserted from the 

 shell inferiorly and mo ved rapidly behind, as it strikes against 

 the bottom. This locomotion has sometimes a distiuctly jerking 

 character. Often, by repeated strokes of the tail, the shell will 

 be turned round several times in succession. and may thus get 

 rat her deeply immerged into the loose muddy deposit of the bottom. 



b. The Partial Motion. 



Under this head I propose to describe the several movements 

 performed by the different parts of the body and its appendages. 

 as observed in the living animal when at rest. 



Movements of the Shell. — The val ves of the shell are mo ved 

 in two manners. being opened and closed. The opening of the 

 valves, whereby the free edges become more or less widely re- 

 moved from each other, always takes place when the animal is 

 about to move, and is also in most cases observed when the animal 

 is restiug on the bottom. This movement is not effected by 

 the action af any muscles, but is simply caused by the mode in 

 which the two valves are connected dorsally, and partly by the 

 elasticity of the valves themselves. The motion of the shell 

 observed on the opening of the valves cannot therefore properly be 

 termed a movement in the same sense as the other movements 

 of the animal, since it is due, not to muscular action, but to 

 the relaxation of the muscle per forming the contrary movement. 

 The latter movement, whereby the opened valves are again closed f 

 in such manner that their free edges become more or less com- 

 pletely applied against each other, is performed by the aid of 

 the strong adductor muscle of the shell, which traverses the max- 

 illar segment and affixes itself to each val ve at a short distance 



Vid.-Selsk. Forh. 1887. No. 1. 3 



