368 OILMAN A. DREW. 



67, h.). It is in the form of a bent spindle, the two ends of 

 which communicate with the blood-spaces of the gill. The 

 larger median portion arches dorsally and surrounds the 

 intestine. Anterior and posterior aort^e leave the heart, but 

 no attempt has been made to follow them, until the adult 

 stage is reached. 



For a considerable time after its formation there is no 

 appreciable change in the heart. About the time that the 

 eighth pair of gill plates are formed it begins to be separated 

 into ventricles and auricles. The auricles are at first very 

 small and narrow. They extend ouly a short distance from 

 each gill, and are separated from the ventricle by slight 

 constrictions. There has been no chauge in the relative 

 positions of the heart and intestine. At a slightly later 

 stage, when the gill has about ten pairs of plates, the ven- 

 tricle of the heart begins to change its shape. This seems to 

 be due to the growth of the kidneys, which push anteriorly 

 ventral to the pericardium. As the kidneys grow, the two 

 sides of the heart are pushed dorsally, while the middle part 

 of its ventral wall is held in its original position by the 

 intestine. In this way the ventral wall is pulled out into a 

 sort of trough in which the intestine lies (fig. 68, h.). Con- 

 tinued growth deepens the trough until it is considerably 

 deeper than the intestine is wide. The heart gradually 

 closes in, dorsal to the intestine, at the anterior and posterior 

 ends of the trough, until it becomes free from the intestine, 

 and lies dorsal to it (fig. 69). This is a very slow process, 

 and is not completed until after the animal has reached 

 sexual maturity.-^ 



The adult heart consists of a ventricle and a pair of auri- 

 cles, separated from each other by constrictions that are 

 much deeper on the dorsal than on the ventral surface 

 (fig. 69, h.). The openings between the auricles and the 

 ventricle are so small that they must be quite obliterated 



1 Every specimen of Nucula proxiina that 1 liave examined lias its 

 lieart perforated by the intestine. The specimens are all of good size, and 

 many of them are the same ones from which I obtained eggs and sperm. 



