STUDIES ON THE NERVOUS SYSTEM OF CRUSTACEA. 467 
runs down the cord to the sixth abdominal ganglion. Here 
the fibre divides into several branches (fig. 5, A (a) Br), but 
_I have never obtained complete staining of these. 
According to Retzius,! in the adult Astacus the branches of 
these fibres pass directly into the nerves which leave the 
posterior end of the ganglion. If this be the case, the elements 
must be placed in a class by themselves, as they serve the 
purpose of putting some organ at the posterior end of the 
abdomen into direct communication with the brain. This is 
rendered more probable by the fact that in their course down 
the ganglionic cord the fibres give off no collateral branches, 
or at any rate none have ever stained, and I believe none to 
exist. In the brain, however, a few branches go to the neuro- 
pile (fig. 2), the most important being at the angles of decus- 
sation. These fibres, both in their narrow portion in the brain 
and in the broad portion in the thorax and abdomen, often, 
under the influence of the reagent or from some other cause, 
assume the beaded form so frequently observed in nerve-fibres. 
This pair of elements evidently corresponds to the pair of 
so-called giant fibres in the adult. The two fibres stain very 
readily in embryos at all stages, but the cells are a little more 
difficult to demonstrate. JI have, however, traced the fibres 
into them with absolute certainty on so many occasions that 
there can be no doubt as to their position and significance. 
They are often best seen by examining the brain, after the 
staining is complete, from the ventral surface. Fig. 6 repre- 
sents such a view. 
A (a) Ant II.—This element commences with a cell in the © 
ganglion of the second antenna. After leaving the cell the 
fibre gives off two stout branches to the neuropile, and then 
passes through the transverse bridge behind the cesophagus 
to the opposite side. Here it turns upwards, gives off a branch 
to the neuropile of the brain, and then turning again down- 
wards, travels along the ganglionic cord. 
This fibre has been traced with certainty as far as Abd. 5, 
1 Retzius, “Zur Kenntniss des nervensystems der Crustaceen,” ‘ Biol. 
Untersuch.,’ neue Folge i, 1890. 
VOL. 36, PART 4,—NEW SER. Ha 
