497 



the shell after it. With Yoldia these movements follow each other 

 in quick succession, and enable an animal to bury itself with great 

 rapidity. With Nucula the movements are not so rapid, but they are 

 very effective. Animals placed on their sides on mud, right themselves 

 without trouble, and quickly disappear. Yoldia executes many move- 

 ments of leaping that Nucula seems never to make. 



bg' 



Fig. 2. Adult specimen of Nucula delphinodonta, represented as seen from the 

 right side. Reconstructed to show internal organs. Fully grown specimens are 4 mm 

 long, aa anterior adductor muscle, bg byssal gland, eg cerebral ganglion. / foot. 

 g gill, h heart, int intestine. Ip labial palp, oes oesophagus, ot otocyst. pa posterior 

 adductor muscle, pap palp-appendage, pg pedal ganglion, sto stomach, vg visceral 

 ganglion. 



Although many specimens have been kept under observation for 

 long periods of time, and although rough and smooth bottomed aquaria, 

 aquaria containing sand, and aquaria containing the mud in which the 

 animals normally live, have been used, no cases of creeping have been 

 observed. N. delphinodonta possesses a very large foot , and the 

 almost spherical shell is easily turned over and thrown about, but the 

 movements of the foot are always such as are used in burrowing. It 

 seems very hard to imagine that the foot could possibly be used as 



