642 journal of Comparative Neurology and Psychology. 



some much smaller worms found near Wood's Hole, Mass., but 

 the shrinkage of the fibrous parts of the brain was so great that the 

 sections were useless for the study of commissures. 



The observations recorded in this paper are based upon the 

 study of the large, well extended worm fixed in 95 per cent alcohol. 

 Only the head has been studied. This was cut in sections about 

 5/< thick, and stained on the sHde with Ehrlich's haematoxyhn, 

 followed by ammonia alcohol, and aqueous eosin. 



3. THE COMMISSURES. 



a. Historical review.- — The brains of different species of Cere- 

 bratulus have been described by a number of observers, notably 

 HuBRECHT (1887), Burger (1895), ^^^ (i^95)' ^"<^ Mont- 

 gomery (1897). All describe a dorsal commissure, and, immedi- 

 ately posterior to it, a broad stout ventral commissure, the first 

 ventral commissure, but there is great difference of opinion in 

 regard to the number and the character of any additional posterior 

 commissures. 



Hubrecht (1887, pi. xiv, fig. 5) figures that part of the brain 

 of Cerebratulus parkeri which is posterior to the dorsal and the 

 first ventral commissure. There are three commissures {c.o.) 

 between the vagus, or CESophageal, nerves {v.a.)y and the dotted 

 lines {c.tr.w.) represent what Hubrecht terms the ventral meta- 

 meric connectives of the brain. In the text, p. ']'], he states, '^the 

 lower brain lobes are united by the ventral commissures, separated 

 by a very short distance, till close up to the massive ventral com- 

 missure that has been hitherto regarded as the only ventral com- 

 missure between the brain lobes. The thin commissures just 

 described are, however, not directly connected with the fibrous 

 core of the brain lobes, which is on the contrary directly continued 

 into the massive inferior commissure, but they seem to derive 

 their fibers from the outer cellular coating of these lobes. They 

 pass underneath the two vagus stems, where these spring from the 

 lower brain lobes, and where these are in their turn in front of the 

 mouth united by transverse commissures." The brain of Dre- 

 panophorus lankesteri (pi. ix, fig. 10) has a series of ladder com- 

 missures between the lateral nerves. 



Burger (1895, taf. 10, fig. 8) figures the brain of Cerebratulus 

 marginatus, in which only the dorsal and the broad ventral com- 

 missure are shown. In the text, however, he describes three com- 



