18 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



the ganglia themselves, lie just posterior to the ventral side of the 

 anterior adductor muscle. Each ganglion is almost circular in 

 transverse section and gradually tapers posteriorly into the cerebro- 

 visceral commissure. So gradually does this tapering take place, that it 

 is impossible to determine where the ganglion ends and commissure 

 begins. Even in structure there is only a difference in degree, there 

 being proportionally more nuclei in the ganglion than in the 

 commissure. Posteriorly each commissure gives place, in the same 

 gradual way, to a visceral ganglion. The visceral ganglia, vg., lie 

 anterior to the ventral side of the posterior adductor muscle, and almost 

 equal the cerebral ganglia in size. The commissure connecting these 

 ganglia is also very broad and, like the cerebral commissure, tends to 

 share the structure of ganglia. 



The pedal ganglia, pg., are rounded and fully as large as the 

 cerebral ganglia. They lie very close together and are connected by a 

 very broad commissure. 



The cerebro-pedal, unlike the cerebro-visceral commissures are not 

 surrounded by nuclei and show ordinary commissural structure. 



Although a great many specimens of widely differing ages have 

 been examined, the double origin of each cerebro-pedal commissure 

 from the cerebral ganglia has not been observed in Yoldia.* Neither 

 is a separation into cerebral and pleural ganglia evident. It is true 

 that the cerebral ganglia are not quite smooth and, in some places, the 

 outer layer of nuclei dips into the fibrous layer, but it seems possible 

 that this may be accounted for by the origin of nerves in the immediate 

 vicinity. 



Each cerebral ganglion gives rise to several nerves. A large nerve 

 passes forward, under the anterior adductor muscle, branches, and 

 supplies the anterior portion of the corresponding lobe of the mantle. 

 Another large nerve passes down to the dorsal margins of the palps, 

 follows them posteriorly and is finally continued the whole length of 

 the palp-appendage. Of the smaller nerves one supplies the anterior 

 adductor muscle and another passes to the body wall. 



Each visceral ganglion gives rise anteriorly to a small osphradial 

 nerve, and posteriorly to a nerve that branches almost immediately. 

 Sometimes these branches originate as separate nerves. One of the 



* There seems to be no doubt that this commissure does have a double origin in 

 Nucula proxima, Say, and N. delphinodonta, Mighels. 



