654 "Journal of Comparative Neurology and Psychology. 



sections behind the ending of the third pair. The cells are one 

 section thick, with merely traces in the next, and measure in 

 width, the right cell 26//, the left 23/<. The fourth pair lies between 

 the third and fourth ventral commissures, but the position in the 

 section of the two cells is not quite the same: the left cell is on the 

 periphery of the left brain lobe medio-ventral to the left oesopha- 

 geal nerve, the right cell is me dio-dorsal to the right oesophageal 

 nerve and just beneath the dorsal blood vessel. A second large 

 cell, 25j« wide, fig. 4, uc, presumably of the fourth type, lies along- 

 side of the right hand cell of this pair, and extends through two 

 sections with traces in a third. The tubule of this second cell, 

 together with that of the right hand cell of the pair, may be traced 

 downward through the right oesophageal nerve into the right 

 brain lobe. The line D F, fig. 4 indicates the dorso- ventral axis 

 of the oesophageal nerve. 



Fifth pair. — The cells of the fifth pair, fig. i, are not quite in the 

 same frontal plane, the left cell beginning three sections, the right 

 cell four sections behind the fourth pair. This is the second 

 largest pair of neurocord cells; the left cell is two sections thick, 

 and has a width of 26/i, the right cell is two sections thick, with a 

 trace in a third section, and is 25/^ wide. This pair is found just 

 anterior to the fourth ventral commissure, and the position in the 

 section of the two cells is similar, both lying on the medial side of 

 the ventral brain lobes, and medio-ventral to the oesophageal 

 nerves. 



Sixth pair. — The cells of the sixth and last pair are in the same 

 section, ten sections behind the ending of the fifth pair. This is 

 the smallest pair, as each cell is present in but one section and 

 has a width of only 2i,«. They are situated posterior to the sixth 

 ventral commissure, fig. i, but their position in the section is 

 asymmetrical, the right cell being on the lateral surface of the right 

 oesophageal nerve, between the nerve and the right brain lobe, the 

 left cell on the medial surface of the left oesophageal nerve. The 

 structure of both cells conforms to that of the ganglion cells of the 

 fourth type. 



CONCLUSIONS AND SUMMARY. 



The present investigation has shown that the ladder-like brain 

 of Cerebratulus lacteus is, in the number of its commissures and 

 neurocord cells, a more complex structure than w^as heretofore 

 supposed. 



