37. 



CIECULATORY SYSTEIl. 



The heart, figure 4, h.^ is situated Just posterior to the 

 beaks of the shell, and verv near the dorsal margin. The cavity 

 of each auricle is connected with the blood spaces of the corres- 

 ponding gill, through a slit in a muscular partition that pro- 

 jects dorsal I7 into the cavity of the auricle. The arrangement 

 of this partition is such as to tend to stop the backward flow of 

 blood into the gills. The ventricle is perforated bv the in- 

 testine. "Where the ventricle joins each auricle, there is a 

 marked constriction and, internally, a muscular septum perforated 

 by a single opening. When the ventricle contracts, this septum 

 probably contracts and more or less completely closes the opening 

 in it. 



Two blood vessels leave the ventricle, one anteriorly 

 on the left side of the intestine, the other posteriorly, beneath 

 the intestine. 



EMBRYOLOGY. 

 The eggs of Y. limatule, are abo\it .15 m m. in diameter, 

 of a chocolate brown color and very opaque. They are laid f r& e 

 in the water and are not encumbered by any kind of envelope. The 

 polar bodies are lost soon after they are formed. The first 

 cleavage, which tastes place about two hours after the eggs are 



