498 



a creeping organ. Its special function, for which its structure so 

 admirably fits it, is for burrowing in soft mud. 



The foot of Yoldia is attached to the shell by four pairs of 

 muscles. One pair of these muscles is posterior, and the other three 

 pairs are anterior. The posterior foot-muscles, Fig. 1 pfm, are in- 

 serted on the shell just in front of the posterior adductor muscle, 

 and very near the bases of the teeth. They are very large, and send 

 all of their fibers forward, along the sides of the foot, to be di- 

 stributed to its anterior and ventral portions. Their function is to 

 retract the foot. The anterior foot-muscles, afm, are inserted, together, 

 on the shell, just behind the anterior adductor muscle, and very near 

 the bases of the teeth. Of the three pairs, the most anterior passes 

 ventrally and posteriorly, along the sides of the foot, and is dis- 

 tributed to its posterior portion. The middle pair passes ventrally, 

 between the pair just mentioned, and is distributed to an intermediate 

 portion. The posterior pair passes ventrally and anteriorly between 

 both of the preceding pairs, and is distributed to the anterior and 

 ventral portions. 



In Nucula the same muscles are present but the anterior ones 

 are not as plainly separated from each other. 



Byssal glands, Figs. 1 and 2 bg, are present in all of the forms, 

 but they do not seem to be functional in any of them. 



Alimentary Canal. 



The position of the alimentary canal of Y. limatula is shown 

 in Fig. 1, and its position for N. delphinodonta is shown in Fig. 2. 

 With N. proxima the arrangement is much the same as with N. del- 

 phinodonta. I find no well marked oesophageal pouches in any of 

 the forms. The hearts of Y. limatula and N. proxima are perforated 

 by the intestines. The heart of N. delphinodonta lies above the in- 

 testine. 



In all of the forms the digestive glands are more extensive on 

 the left than on the right side. This is true from an early period, 

 long before the loop of intestine that comes to lie on the right side 

 has been formed. It seems possible that the relative size of the glands 

 may determine the position of the intestine. 



Labial Palps. 

 The labial palps of the adult animals are very large, Figs. 1 

 and 2 Ip. Each united pair is suspended from the body wall by a 

 thin membrane that contains a few muscle - fibres. Each outer palp 



