THE LIFE-HISTORY OP NUCULA DELPHINODONTA. 379 



just mentioned, and is distributed to the anterior and ventral 

 portions of tlie foot. 



All of these muscles are closely bound together by their 

 own fibres and by interlacing fibres, so that many movements 

 occur that cannot be explained by direct pulls of one or more 

 muscles. It should be remembered that the attachments of 

 the fibres are all along the sides of the foot, and that many, 

 if not most of the muscle-fibres, pull from one part of the 

 body-wall to another, without changing the relation of the 

 body to the shell. Thus the muscular side flaps of the foot 

 can be spread apart after the animal has been removed from 

 the shell. 



Between the muscles, loose connective tissue and large 

 blood-spaces occur. Many of the movements, especially 

 those that result in the protrusion of the foot, seem to 

 depend on the action of muscles on the fluids of the body, 

 more especially upon the blood contained in the spaces of the 

 foot. By obliterating some channels and forcing blood into 

 others, different results may be obtained. 



The muscle-fibres that are attached to the shell along the 

 ventral border of the genital mass and liver are distributed 

 to the body-Avall. They are not as numerous as they are 

 in Yoldia. I have found no indication of a special muscle 

 at the posterior end of each series, as is the case with 

 Yoldia (3). 



The heart is largely made up of interlacing muscle-fibres. 

 Bach auricle is separated from the ventricle by a constriction 

 (figs. 68 and G9, h.) . It seems probable that, when the ventricle 

 begins to contract, the contraction of the muscles in these 

 constrictions closes the openings between the ventricle and 

 the auricles so that the blood cannot flow back into them. 

 Where the auricles join the blood-spaces of the gills and 

 mantle lobes, the muscles probably act in the same "way. 



There are some muscle-fibres in the suspensory membranes 

 of the gills that probably contract at intervals. The opaque 

 shells make it impossible to watch the movements of the gills, 

 but it will be seen that such movements as are made must 



