870 GILMAN A. DREW. 



probably originally a double organ, and that upon uniting in 

 the median line it has taken up the various positions in regard to 

 the intestine. Grobben (5) considers the single heart primi- 

 tive, and thinks that the double condition is the result of 

 changes in the position of retractor muscles. Pelseneer (13) 

 and others, depending largely upon the position of the heart 

 in Nucula and Area, have considered the dorsal position of 

 the heart to be the primitive position. Stempell (17) rightly 

 holds that the ventral position of the heart of Malletia 

 chilensis destroys the oundation of Pelseneer's reasoning, 

 inasmuch as Nucula and Malletia are closely related forms. 

 Stempell apparently considers the perforated heart to be 

 the most primitive. From this position the heart may become 

 dorsal or ventral to the intestine by a comparatively simple 

 process. 



The development of the heart of Nucula seems to indicate 

 that the perforated heart is more primitive than the dorsal 

 heart in this gi'oup. While, as Stempell holds, it seems most 

 reasonable to consider a perforated heart that may become 

 either dorsal or ventral by comparatively simple changes as 

 more primitive than either a dorsally or a ventrally placed 

 heart — where, in order to reach the opposite extreme, the 

 heart would have to enclose the intestine, and then become 

 free on the other side, — there is still nothing to prove that the 

 ventral position of the heart is not primitive. The develop- 

 ment of the heart of Malletia would accordingly be of con- 

 siderable interest. 



As neither the pericardium nor the heart of this Lamelli- 



branch seems to be formed as a paired structure, there is 



nothing here to further the view of Thiele (19) that the 



position of the heart in regard to the intestine depends 



simply upon the position of two lateral hearts, that may, as a 



matter of convenience, fuse dorsally, ventrally, or around the 



intestine. 



Nervous System. 



The cerebral ganglia are formed in direct contact with the 

 apical plate. The cells from which they originate can first 



