426 MONTGOMERY. [Vol. XIII. 



sents a somewhat different structure in different portions of 

 the lateral chord: thus anteriorly (Fig. 35 rt^) the finer nerve 

 tubules preponderate in number, and but few neurochords 

 occur, while more posteriorly (Fig. 35 b, c) the neurochords 

 increase in number, which is due to their dichotomic divisions 

 as well as to the accession of new neurochords from the cells 

 IV placed along the chord, these cells becoming more numerous 

 posteriorly. 



Thus I have found no nearly central bundle of neurochords, 

 such as Burger ( 9ib) has described, but rather individual neuro- 

 chords of various diameters as well as the smaller nerve tubules 

 (of cells II and III) throughout the whole diameter of the 

 lateral chord. Although I did not observe a nerve tube in the 

 lateral chord, such as has been described for Linetcs, I would 

 not thereby maintain the absence of such a structure in C. 

 lacteus, since it might here easily be confounded with a neuro- 

 chord. 



VI, Brain Commissures, Oesophageal Nerves. 



The large ventral commissure of the brain which had been 

 distinguished by the pioneers of nemertean anatomy, Delle 

 Chiaje (1823) and Quatrefages (1846), has been supposed by 

 all succeeding investigators to be the only ventral commissure. 

 This large commissure may be termed, however, the first ven- 

 tral commissure to distinguish it from two posterior commissures 

 which are described here for the first time, and which may be 

 termed the second and third ventral commissures respectively. 

 In Cerebratulus lacteus the second ventral commissure arises 

 from the fibrous core of each ventral lobe, just ventral to the 

 oesophageal nerves, and one section behind the first pair of 

 neurochord cells of the brain. It has scarcely one-fifth the 

 diameter of the oesophageal nerve at this point, and, though it 

 apparently has no special neurilemmatic envelope, is separated 

 from this nerve by the neurilemmatic sheath of the latter. The 

 nerve tubules of which this commissure is composed (probably 

 those of cells II and III) are derived from the fibrous core of 

 either ventral lobe. In Lineits gcsserensis (Fig. 43) this second 

 ventral commissure {Comm. 2) has the same position and struc- 



