438 
PAUL PELSENEER. 
cipally horizontal longitudinal ones. I have thus been obliged 
to use a great many specimens. 
The brain of Apus is flattened and its shape quadrangular 
(fig. 5). It is situated obliquely in relation to the longitudinal 
axis of the body (fig. 3, c), and if one takes account of the dorsal 
flexure of the nervous cord in front of the oesophagus, 1 the 
brain’s superior surface is in fact the ventral (infra-neural), and 
the posterior margin out of which come the optic nerves is 
the anterior margin. 2 
The ganglionic cells are accumulated on the ventral surface 
of the brain ; among these one can observe two distinct shapes : 
first, small cells round or oval, forming a single layer on the 
ventral surface (figs. 6 and 7, c s.) and a thicker mass on the 
posterior edge of the brain (fig. 5, ap); secondly, large pyriform 
cells which are found in certain parts of the brain. 
I have drawn, cell by cell, the whole series of transverse 
sections, so that I could clearly determine by superposition the 
disposition of the ganglionic cells. 
On the anterior part of the brain the large cells form a very 
thick mass (fig. 4, a) at the beginning of the optic nerves. 
That mass extends backward without transition, and assumes a 
charactei'istic shape. In a transverse section taken a little 
in front of the middle of the brain (fig. 6) the large cells, the 
prolongations of which go toward the interior of the brain, 
are divided into two symmetrical groups, distinct though joined 
(g 1) ; they are the primitive cephalic ganglia. 
If one goes on observing the successive transverse sections 
from before backwards, the primitive cephalic ganglia will 
be seen ending, without any continuation, a little after the 
middle of the brain. Towards the edge of the latter, and princi- 
pally towards the ventral surface, a second pair of groups of 
large pyriform cells will appear (fig. 7, g 2). This group pro- 
1 Such a flexure is found in many Crustacea. 
2 I shall hereafter speak of the brain of Apus morphologically, not topo- 
graphically, i.e. the surface which is superior when the brain is in situ will 
be called ventral surface ; the posterior margin, by dissection, will be the 
anterior margin, and vice versa. 
