THIi] LIFE-HISTOEY OF NUCULA DELPHINODONTA. 845 



the surface of the mud. Observations made on specimens 

 kept in dishes of sea Avater in which there was no mud show 

 that individuals of this species execute movements very 

 similar to those executed by Yoldia (1), but that in all cases 

 they are much more deliberate (3). Leaping movements are 

 absent, but slow thrusts with the flaps extended may fre- 

 quently be observed. In former publications attention has 

 been called to the characteristic movements of the foot, and 

 they need not here be redescribed (1 and 3). As in the case 

 of other members of this group, the movements of burrowing 

 are very effective. The somewhat spherical shape of the 

 shell, and the relatively large size of the foot, make it 

 possible to raise the shell from the bottom of a dish, and 

 occasionally to keep it balanced for a few seconds over the 

 expanded foot. My observations lead me to believe that the 

 animals never creep. 



As in Yoldia, the foot is supplied with complicated and 

 powerful muscles (1 and 3). It is attached to the shell by 

 three pairs of muscles, and by a few fibres that lie ventral to 

 the genital mass and liver. The posterior pair of foot 

 muscles is very powerful. These muscles are attached to 

 the shell at the bases of the teeth, just anterior to the pos- 

 terior adductor muscle, and extend along the sides of the 

 foot in an anterior and ventral direction. They are the 

 powerful retractor muscles of the foot. Fibres from them 

 are extended into the muscular flaps, and are important in 

 spreading them apart. 



The two anterior pairs of foot muscles correspond to the 

 three anterior pairs of foot muscles in Yoldia. They are in- 

 serted on the shell close together along the bases of the 

 teeth, just posterior to the anterior adductor muscle. The 

 most anterior pair has much the same distribution as the two 

 anterior pairs in Yoldia, and in some cases each muscle seems 

 to be slightly separated into two near its origin. They spread 

 out along the sides of the foot, and are distributed to its 

 posterior and ventral portions. These muscles seem to be 

 closely connected with the muscle-fibres that are attached 



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