Turner, Mushroom Bodies of the Crayfish. 326 
back of it to form the ventral chain. At first it is composed 
of a superficial and a deeper ganglionic layer. The cells of the 
ganglionic layer, by fusion, form the nerve cells. From the 
inner surface of the nerve cells ramify fibers forming the fibrillar 
substance. In the embryonic chain, each neuromere is lodged 
in a separate somite, the transverse commissures are large and 
double and the narrow longitudinal connectives are completely 
surrounded by ganglion cells. 
In the adult the morphology of the ventral chain varies 
considerably in different types. In some cases the longitudinal 
commissures are long, but in others the ganglia are crowded 
together and lie in front of the somites innervated. An elon- 
gated nervous system does not represent a primitive condition, 
for in the embryo the longitudinal connectives are quite short. 
The compact chain does not represent so much a primitive stage 
as a stage in which the longitudinal commissures of the embryo 
have been effectually shortened. 
Brain of Insects. The brain of the insects, which lies in 
front of and above the oesophagus, is composed of a proto- 
cerebrum, a deutocerebrum and a tritocerebrum—three neuro- 
meres which correspond to three primitive somites. 
The protocerebrum is composed of a median portion and 
two lateral optic ganglia. These optic ganglia, which are situ- 
ated between the compound eyes and the median portion of 
the protocerebrum, are composed of three ganglionic masses. 
The median portion of the protocerebrum is composed of two 
lobes intimately connected at the meson. It contains: the 
mushroom bodies, the central body and the protocerebral 
bridge. | 
The mushroom bodies are united by commissural fibers to 
each other, to the optic ganglia of the same side, to the central 
body and to the olfactory lobes. 
The central body, which is composed of fibers, has fiber 
connections with the cerebral lobes, the mushroom bodies, the 
optic ganglia and the olfactory lobes. 
The deutocerebrum is composed of two hemispherical ol- 
factory lobes. In addition to being united to its fellow by a 
