1916.] NATURAL SCIENCES OF PHILADELPHIA. 483 



inserted in practically the same fashion with the marginal mantle 

 nerve. 



The osphradial ganglion is a well-defined elongated body situated 

 far forward on the left side of the mantle cavity. The nerve leaving 

 its posterior border pursues a parallel course with the anterior 

 mantle ring connective and enters the supra-intestinal ganglion. 

 Certain species of the genus Crepidula are reported. 4 to po-~e-< a 

 sinistral zygoneury wherein the supra-intestinal ganglion is united 

 to the pleural of the same side. I have found no evidence of its 

 existence in C. arfunca. 



Turning now to the development of the nervous system, we find 

 that shortly after the first appearance of the foot the ectodermal 

 cells immediately in front of the lateral angles of the mouth opening 

 commence to elongate, and each area rapidly differentiates into a 

 well-defined, probably sensory ridge and the tentacle. Associated 

 with these external developments there is a migration of ectodermal 

 cells from the region of the ridge and tentacle, slight at first, but 

 rapidly increasing, and finally ending, if one can rely on sections, 

 at the time when pigment first makes its appearance in the eye. 

 These migrant cells form the cerebral ganglia, more or less rounded 

 at first, and largely obscured by the heavily staining overlying 

 ectoderm. 



Synchronous with the development of the cerebral ganglia the 

 pedal ganglia arise. With the exception of a median, ciliated, slender, 

 wedge-like area, the base facing anteriorly, the cells of the foot likewise 

 become elongated, whereupon cells migrate inwards, forming two 

 large, diffuse masses in contact posteriorly where the commissure 

 subsequently arises. The four ganglionic bodies thus existing at 

 this time appear to be the product solely of migrating elements, 

 since sections give no indication of cell division after the cells have 

 left the ectodermal layer. The same is also true of all the other 

 ganglia now to be considered. 



In whole mounts and in sections the cells along the posterior 

 border of each cerebral ganglion can be seen in the late stages of its 

 development to assume an elongated form and to develop fibres 

 which extend posteriorly. In some instances the strands thus 

 produced form two distinct bundles at the outset, one of which 

 passes into the pedal ganglion, while the other at a slightly later 

 stage joins with the pleural. In other instances one bundle first 



4 Lang's Lehrbuch, p. 213. 



