646 'Journal of Comparative Neurology ajid Psychology. 



il/ji. This is a brain commissure, as the roots come from the 

 cellular sheath of the brain lobes, and it is clear and distinct, 

 though delicate. As the central part is at a more ventral level 

 than the roots, a wide V is formed, which gives the commissure 

 a very distinctive appearance. A few cells of type I are scattered 

 along the surface, but there is no continuous layer. Thiscommis- 

 sure is intermediate in position to the first two pairs of neurocord 

 cells, and evidently corresponds to the second ventral commissure 

 .described by Montgomery (1897) for Cerebratulus and Lineus. 



Third ventral commissure.- — The third ventral commissure, fig. I, 

 5, is seven sections posterior to the second, and extends through 

 three sections. It is a well defined brain commissure, as the roots 

 may be traced into the cellular sheath of the brain lobes. On the 

 right side, fig. 3, two roots are clearly distinguishable, passing 

 toward the brain, one dorsal and one ventral to the right oesopha- 

 geal nerve, the latter entering the fibrous core. On the left side, 

 only one root is seen, ventral to the oesophageal nerve. Like the 

 second, this commissure also forms a broad V. The central part 

 lies at a more ventral level and is considerably broader than the 

 roots, measuring 24.fi dorso-ventrally in the broadest part. The 

 anterior border is thickly beset with ganglion cells of typs I. 

 This commissure is here described for the first time in Cerebratulus 

 but seems to correspond, except in its distance from the second, 

 with the third ventral commissure described by Montgomery 

 in Lineus sp. 



Fourth ventral commissure. — The fourth ventral commissure, 

 fig. I, ./, is situated eight sections posterior to the third. It is 

 three sections thick and has a dorso-ventral measurement of 42/^. 

 The fibers of this commissure are derived from the cellular sheaths 

 of the brain lobes and run from side to side in nearly a straight 

 line at the level of the ventral surface of the oesophageal nerves. A 

 few cells of type I are scattered along the surface of the commissure. 



Fifth ventral commissure. — The fifth ventral commissure, fig. 

 5, comes from the fibrous core of the brain, and lies four sections 

 behind the fourth. The central mass is one section thick and has 

 a dorso-ventral measurement of 50//. This is the only commissure 

 about which there is any doubt; the roots are clear and distinct, 

 and extend through several sections, fig. 5,5, the right root measur- 

 ing in width 2i,«, the left slightly less, and there is a short central 

 mass in one section, fig. 6, §c, but the connections between this 



