340 JouRNAL OF COMPARATIVE NEUROLOGY. 
cated in the eye-stalk and connected to the brain by what is 
known as the optic nerve. Again, in the insects, trachez sur- 
round and even penetrate the brain; but, in the decapod crus- 
tacea, there are no trachee to complicate matters. These two 
minor points of difference do not militate against the idea that 
the brains of insects and crustacea are homologous organs. 
It has been demonstrated by VIALLANEs (’93) and others 
that the brain of the crayfish, like that of the insects, is com- 
posed of the following three regions: protocerebrum, deuto- 
cerebrum, tritocerebrum. Although agreeing in this matter 
with VIALLANEs, and occasionally referring to each of these 
regions as such, yet the author does not propose to bound 
accurately these regions, for the simple reason that in the adult 
there are no sharp demarcation lines between these segments 
and any attempt to locate absolute boundaries is sure to be, not 
only artificial, but also confusing. 
Viewed from its cephalic aspect, the brain of the crayfish 
(Cambarus) is composed of a median portion and two lateral 
swellings. This median portion easily differentiates itself into 
a dorsal and a ventral region.’ The lateral swellings contain the 
mushroom bodies and the optic and olfactory lobes (fig. 1, O/. 
Te SOP L:.). 
1 Students of invertebrate neurology will recall that KRIEGER (’88) divides 
the median portion of the crayfish brain into anterior and posterior swellings. 
What KRIEGER has called anterior is here called dorsal and what KRIEGER has 
called posterior is here called ventral. ‘The reason for this change in nomen- 
clature will now be given. The brain of the decapod crustacea is so located in 
the head that that portion from which the optic nerve arises is directed upwards 
(dorsad), while the portion from which the antennulary nerve arises is directed ; 
forwards (cephalad). This may be easily demonstrated, either by dissections 
of Caméarus or by transverse sections through the entire head of Palemon. On 
viewing a brain that has been removed from the body of a decapod, one is 
inclined to call that portion from which the optic nerve springs the cephalic 
end. I was misled until, on examining some sections through the entire head 
of falemon, I found that the dorsal portion of the brain of Palemon agreed in 
all essentials with what I, along with others, had been calling the cephalic end 
of the brain of Caméarus. This led to a careful examination of the Camébarus 
brain zz sztuz. Then I discovered that the optic nerve arises from the upper 
(dorsal) portion of the brain. 
