472 F. W. GAMBLE AND J. H. ASHWORTH. 



The brain of A. cristata is composed of the same elements 

 as that of the two species just described. It is, however, 

 somewhat shorter, the anterior and posterior lobes are more 

 approximated, and the intermediate region correspondingly 

 reduced. 



In A. ecaudata and A. Grrubii the brain is wide from 

 side to side, and narrow from before backwards. The layer 

 of ganglion cells on its upper surface is produced into short 

 blunt processes, with intervening ccelomic and vascular 

 spaces. It contains the same two chief kinds of elements as 

 in A. marina, but the neuroglia is much more strongly 

 developed, and consists of multipolar cells lying in the fibrous 

 part of the brain, and sending processes in all directions. 

 The neuropile, especially in the brain of A. Grubii, is not 

 so fine as in, e. g., A. Claparedii. 



The ganglionic layer of the brain in all species is in contact 

 with the epidermis of the prostomium. In the "marina" 

 section this occurs along the entire dorsal surface; in the 

 ^'ecaudata" section it is affected by the ganglionic out- 

 growths, and is not so intimate. In this respect and in histo- 

 logical structure the brain of the latter section of the genus 

 Arenicola resembles that of Clymene (Maldanidae) as de- 

 scribed by Eacovitza; that of the former is more closely similar, 

 however, to the brain of Mastobranchus (Capitellidse). 



Cerebral Nerves. — The anterior brain lobes, in addition 

 to the oesophageal commissures, give off nerves to the anterior 

 end of the prostomium, to the upper lip, and the eversible 

 buccal mass. The latter are probably connected with the 

 oesophageal and gastric plexuses. lu A. ecaudata and 

 A. Grubii, as was mentioned above (p. 470), the ciliated 

 grooves are generally supplied by several nerves arising 

 from the simple commissural brain. 



The Growth of the Brain. — The changes which take 

 place in the arrangement of the nervous elements of the 

 brain during the life of Polycheetes, and especially those which 

 occur up to the attainment of the adult characters, have 

 hitherto scarcely been investigated at all. Nevertheless even 



