HAMAKER: NERVOUS SYSTEM OF NEREIS VIRENS. 109 
decapod Crustacea. In the latter, the fibrous part is relatively much 
greater, and the fibres are collected into small bundles forming numerous 
commissures between the various parts of the brain. Since the number 
and size of the nerves leaving the brain of decapod Crustacea is small 
compared with the size of the brain, the increase in the fibrous sub- 
stance of the brain must be due to a greater development of the associa- 
tion fibres of all kinds, including not only fibres which lie wholly within 
the brain, but also those branches of centripetal and centrifugal fibres 
which bring the various parts of the brain into relation with one another. 
This condition is apparently correlated with the increased development 
of the “ mushroom bodies” in Arthropods, as we shall see below. 
4, “ MusHroom Bopigs.” 
The compact masses of small nuclei that lie in the anterior part of 
the brain of Nereis (Plate 3, Figs. 24, 21) have been described by a 
number of writers, who have, however, usually expressed considerable 
doubt concerning their significance. Ehlers (’68) and Schréder (’86) 
describe this structure under the name “ Nervenkérner.” Rohde (’87) 
calls a similar structure in Polynoé and other Polychetes ‘ Nerven- 
kernen.” Retzius (’95) refers to it as a “ Haufen groben Korner,” 
which he says are larger about the periphery of the mass. He thinks 
the larger granules may be cells, but doubts the cellular character of 
the smaller ones. His preparations were stained in methylen blue, but 
showed no processes connected with the nuclei. Haller (’89) discusses 
the nature of these structures at some length, and describes the elements 
as small multipolar ganglion cells. He calls the mass a “ Tentakel- 
ganglion,” and supposes it to be connected with the sense organs of the 
antenne. Racowitza (’96) applies to it a similar term, “ganglion anten- 
naire,” but he does not mean to indicate thereby that the ganglion has 
any direct connection with the antenna. Haller objects to Rohde’s ap- 
plication of the descriptive term ‘‘ Hutpilz ” to these ganglia “ weil sie 
sehr leicht zu einer Verwechselung mit den hutpilzférmigen K6érpern 
am Hirn der Insecten veranlassen diirfte, mit denen aber diese Ganglien 
nichts Homologes aufweisen kiénnen.” 
Notwithstanding this statement of Haller, I think there are good 
reasons for considering this organ as in some degree homologous with 
the mushroom bodies of the insect brain. The resemblance between the 
two appears more strongly, if we compare both with a corresponding 
structure in the brain of the crayfish. On the anterior lateral border 
of the brain of this Crustacean there is a triangular mass of small cells 
