Thompson, Brains of Cerebratulus. 653 



from the fibrous core of the ventral brain lobes, fig. i, «i, and it 

 therefore agrees in position with the single pair of Burger, and 

 the first pair of Montgomery. The cells of the first pair, fig. 2, 

 /«!, r«i, are not symmetrically placed, nor are they exactly equal 

 in size. The right hand cell begins two sections anterior to the 

 left, and extends through four sections, with traces in a fifth; the 

 left cell is present in three sections, with traces in a fourth. The 

 width of the right cell is 30/x and that of the left 27//. The right 

 cell is very conspicuous on account of its great size and its unusual 

 position, on the ventral surface of the right ventral brain lobe, 

 slightly late rad of the median line of the lobe, and just on the border 

 of the cellular sheath. That the width of this cell seems, in fig. 2, 

 greater than the length may be accounted for by the plane of the 

 section, which has evidently cut the cell at an angle of about 40° 

 to its long axis. The tubule runs obliquely but directly upward 

 into the fibrous core of the brain lobe. The left cell is situated on 

 the medio-ventral surface of the left ventral brain lobe, within the 

 cellular sheath, and its tubule may be traced directly into the 

 fibrous core. 



Second pair. — The cells of the second pair, fig. i, lie in the same 

 section, five sections behind the ending of the first pair, and are only 

 one section thick. In width the left cell measures 25//, the right 

 cell 2i/i. The position of the cells is on the medio-ventral surface 

 of the oesophageal nerves, near the periphery of the cellular sheath 

 of the ventral lobes, and between the second and third ventral 

 commissures. 



Third pair. — The neurocord cells of the third pair are not in the 

 same frontal plane, fig. i, fig. 3, In^. The right cell begins 

 three sections, the left cell four sections behind the second pair. 

 Each cell is two sections thick. Their position is near the third 

 ventral commissure on the median side of the oesophageal nerves, 

 the right cell slightly more dorsal than the left and nearer the 

 dorsal blood vessel. In fig. 3 only the left cell is shown, as the 

 right cell ended in the preceding section. The third pair is one 

 of the smaller pairs of neurocord cells, the right cell measuring 

 in width only 21/^, the left cell 23/^; but, in spite of the size, the 

 structure is particularly clear, so that there is no doubt that these 

 cells are of the fourth type. 



Fourth pair and the unpaired right hand cell. — The cells of the 

 fourth pair, fig. i, fig. 4, rw^, are found in the same section, three 



