Thompson, Brains of Cerebratulus. 655 



If we examine the brain of the Metanemertean, Drepanophorus 

 lankesteri, figured by Hubrecht (1887, plate ix, fig. 10) we find, 

 posterior to the dorsal and the thick first ventral commissure, a 

 series of thin ventral commissures between the lateral nerves. 

 The commissures occur at fairly regular intervals, and the adja- 

 cent ones are occasionally connected by irregular fiber bundles. 



In the Turbellaria, in the brain of Planocera graffii, figured by 

 Lang (1884, taf. 31, figs. 3 to 4) posterior to the brain are two stout, 

 and many delicate irregular commissures between the lateral 

 nerves, making an intricate network of fibers, but with a generally 

 ladder-like appearance. Again, in the brain of Cestoplana (taf. 31, 

 fig. 2) there is a continuous crossing and interlacing of fibers 

 between the lateral nerves, as far back as the beginning of the 

 proboscis. The nervous system of Gunda segmentata (Lang 1881, 

 taf. xii, fig. i) is well known on account of the metameric series 

 of commissures between the oesophageal nerves throughout the 

 length of the body. 



The comparison of the brains mentioned above with that of 

 Cerebratulus lacteus as described in this paper leads to the con- 

 clusion that the brain of Cerebratulus, though complex, is probably 

 of a less specialized and more primitive type than has been sup- 

 posed. 



The greater number of neurocord cells distributed over a 

 greater part of the brain is probably also a primitive character. 

 It is known that they are irregularly placed along the lateral cords, 

 and, the more primitive the brain, the closer is the resemblance 

 in structure of the lateral cords and brain lobes proper. 



The presence of the unpaired neurocord cell adjacent to the 

 fourth pair leads me to believe that the number of neurocord cells 

 in the brain is not fixed but variable, and may differ in every 

 individual. The fact that Montgomery found only three pairs 

 of neurocord cells in this same species is additional evidence. 



It is probably also true that the number of the ventral com- 

 missures varies somewhat with the individual. The stouter com- 

 missures, especially those originating in the fibrous core of the 

 brain, would vary least, but the delicate ones, and those derived 

 from the cellular sheath, would be most capable of variation. 



I. Thirteen ventral commissures, posterior to the broad, first 

 ventral commissure, are found in the brain of a large well extended 

 individual of Cerebratulus lacteus. 



