No. 3.] STUDIES ON THE HETERONEMERTINI. 399 



a. Distribution. — In Cerebratubis marginattis and Langia 

 formosa, Burger found one pair of these cells in the ventral 

 lobe of the brain, and a considerable number placed irregularly 

 along the lateral nerve chords. In Drepanophorus and Prosa- 

 denoporiis he found only one pair, situated in the brain, and 

 none along the lateral chords. 



In C. lactetts I have made a more detailed investigation of 

 their distribution, based upon a study of several series of trans- 

 verse sections (of about 5;u, thickness) through an immature 

 individual of approximately 6 in. in length. In this way about 

 four-fifths of the worm was sectioned. Since it was not my 

 intention to determine the total number of neurochord cells in 

 the body, — a number which probably varies in different indi- 

 viduals, — but rather to determine their proportionate number 

 in the two lateral chords and on the dorsal and ventral sides 

 of each chord, it would have been an unnecessary consumption 

 of time to cut the whole animal into thin sections of 5/i thick- 

 ness. Even had this been done, the total number of cells 

 could have been given only approximately, since quite a num- 

 ber of cells occur which are structurally intermediate between 

 III and IV, putting the calculator into the quandary 0^ not 

 knowing whether to count them in or whether to leave them 

 out ; and whichever way he should determine upon, he could 

 not be certain of the correctness of his resulting figures. 



Three pairs of neurochord cells are present in the ventral 

 lobes of the brain (three cells in each lobe). The cells of the 

 first pair (Fig. 24, C. IV) lie close beneath the dorsal blood 

 vessel iD. V.), on the medial sides of the ventral brain lobes, 

 in a frontal plane which cuts the oesophageal nerves about the 

 point where they begin to pass out of the fibrous core of the 

 lobes. Both cells lie in the same frontal plane. This was the 

 only pair found by Burger in the brain of C. marginatus and 

 Langia. The second pair of cells do not lie in one and the 

 same plane : the first cell lies on the right hand, one section 

 behind the first pair of cells, the second cell lies on the left 

 hand, five sections behind the first pair of cells ; thus the cells 

 of the second pair are three sections apart. The third pair of 

 cells are placed two sections behind the most posterior cell of 



