142 MUSCULAR SYSTEM. 



retracted, the trunk of the animal is compressed, the 

 respirat< j emptied, and the whole body is 



retracted ^ the fixed point to which the animal is 



attached *e respiratory currents are produced by the 

 vibratile cilia, disposed on the branchiee, and on the 

 thoracic cavity, as in other acephalous mollusca, and 

 the swimming of some of the aggregate forms, as of 

 the pyrosoma, is effected by the same respiratory cur- 

 rents produced by vibratile cilia. The muscular coat of 

 the tunicata is analogous to the mantle of bivalves, as 

 their cartilaginous covering is the analogue of the 

 shell. 



The conchiferous animals are much more generally free 

 than the tunicata, and some of them possess considerable 

 power of locomotion. The movements are chiefly performed 

 by the foot, which is commonly a lengthened tongue- 

 shaped, muscular organ, capable of being protruded to some 

 distance from the cavity of the mantle, and capable of as- 

 suming a great variety of forms. By this organ the con- 

 chifera attach their byssus, swim at the surface of the 

 water, creep on a solid surface, burrow in sand, or 

 other soft material, and extricate themselves when 

 covered. The foot is composed of muscular fasciculi, 

 which decussate each other in various directions to give 

 it great variety of movements, and water is often ad- 

 mitted into its interior cavity. It is sometimes wanting where 

 the shell is permanently fixed to a spot, as in the oyster. 

 It is generally more or less connected with the dorsal 

 part of the valves, and the fibres of its expanded base 

 embrace almost the whole of the abdominal cavity. 

 The adductor muscles are the active organs by which 

 the valves are closed against the elastic property of 

 the ligament, and they generally consist of one or two 

 thick, short, and strong muscles, which pass straight 

 across the ventral surface of the abdomen, to be at- 

 tached ta the inner surface of both valves. The pecten 

 is enabled to swim backwards by the powerful and 

 repeated action of its adductor muscle on the valves. 



The adductor muscle is large and single in most of the 

 round forms of conchifera, as the pecten, ostrea, anomia, 

 spondylus, but in the more lengthened forms there are 



