STUDIES ON THE NERVOUS SYSTEM OF CRUSTACEA. 35 
bling one another in details of arrangement. Also, in a pre- 
paration of an earlier embryo, in which the fibres B, Th. v, 
and B, Th. vir1, were completely stained, a third fibre was 
observed ending with them in the brain, running close to them 
along the cord, but proceeding backwards beyond Th. vir. 
Its destination at that time remained undetermined, but there 
can be little doubt that it was the fibre of this element, B, Th. xt. 
It will be observed, too, that Th. xr is exactly the ganglion 
in which we should expect to find another element of this class. 
Such elements have already been described (see Pt. I, pl. 36, 
fig. 3) in Th. 11 (with branches to Th. 1 and Th. 111), in Th. v 
(with branches to Th. 1v and Th. vi), in Th. vitr (with 
branches to Th. vir and Th. 1x), and now we find a similar 
one in Th. xi (with branches to Th. x and Abd. 1). In this 
way each ganglion of the thorax must be influenced by these 
elements, which end in a particular region of the brain. 
The series of elements of which the element B, Th. x1, is a 
type resembles in many respects the series of elements de- 
scribed by Retzius,' in Amphioxus lanceolatus, the cells of 
which give rise to the giant fibres in the nerve-cord of that 
animal. These fibres, after leaving the cell and crossing to 
the opposite side of the cord, are described as running forwards 
for some distance, but their ultimate fate was not determined. 
A knowledge of the nature and position of their endings would 
be of great interest. 
PropaBLe Motor Exements or Anterior Roots.—The 
elements E, G, and H (fig. 1. Compare also Pt. I, pl. 35, 
fig. 1), the fibres of which leave the ganglia by the anterior 
nerve-roots, and which have already been described for the 
anterior ganglia, occur also in Th. 1x, x, and xt. 
The elements E (4) and H (e) (fig. 1) resemble in general 
features the element E, but differ from it in the number and 
mode of branching of the fibres which they give off to the 
neuropile. Their characteristic appearance and the situation of 
the cells may be seen from the figure. They occur upon both 
sides in the ganglia from Th. vii1 to Th. x1. 
1 Retzius, ‘ Biol. Untersuch. Neue Folge,’ ii, 1891. 
