236 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



It is there wholly automatic. It is probably at least 

 semi-automatic in the locomotion of worms. 



The locomotion of all wormlike animals, as, for ex- 

 ample, caterpillars, etc., is similar to that described. 



MOLLUSCA. 



Acephala. — In oysters there is only one conspicuous 

 motion, viz., that of closing the valves. This is effected 

 by a large muscle running transversely from valve to 

 valve. When this is cut, as in opening an oyster, the 

 valves fall apart. The purple spot seen on the interior 

 of an oyster shell is the place of attachment of this mus- 

 cle, and the firmer, sometimes tough, portion of the 

 oyster is the muscle itself. When the muscle relaxes, 

 the valves open by means of an elastic substance in the 

 hinge of the valves ; when it contracts, the valves are 

 closed with great force. 



In clams (Fig. 151) there are two of these transverse 

 muscles and an additional locomotive organ, the foot, 



Fig. 151. — Mactra with one valve removed, showing the anterior (am) and 

 the posterior (pm) shell muscles, and the foot (/). (From Gegenbaur.) 



connected with the two transverse or shell muscles. 

 This organ contains longitudinal fibers for retraction, 

 and also oblique and transverse fibers for constriction 

 and consequent protrusion. It is used for locomotion 

 and also for burrowing in the mud. In locomotion it is 

 protruded, takes hold, and then is retracted, and thus 

 draws the body forward. 



