OBSERVATIONS ON THE NERVOUS SYSTEM OF APUS. 433 
Observations on the Nervous System of Apus. 
By 
Paul Pelseneer, B.Sc. 
With Plate XXX. 
The nervous system of Phvllopods recalls, by its appear- 
ance, that of some Chsetopods. The two lateral cords are 
rather distant from one another, and the commissures which 
join the corresponding ganglia are rather long in the anterior 
part. Zoologists agree to recognise that, among the living 
Crustacea, the Phyllopods show the most primitive condition 
of the nervous system, and that this has remained in a rather 
archaic condition. Its study will be, therefore, for the expla- 
nation of some morphological facts, more useful than that of 
the nervous system of superior Crustacea, which has under- 
gone many alterations. 
The anterior part of the nervous system of Apus shows, 
when dissected, the following external appearance (fig. 1) : 
From the upper part of the brain c come the optic nerves 
n o, and from the lower part the two abdominal cords 
c a. From the latter, and rather far behind the brain, 
come, one after the other, the nerves of the two pairs of 
antennae (a i and a ii ). 1 A little further elongated swelling 
1 The two pairs of antennae always exist in Apus ; this fact being already 
stated by Prof. E. Ray Lankester (this Journal, 1881, p. 316). Claus mentions 
the absence of the second pair of antennae as characteristic of the Apusidae 
(‘ Grundziige der Zoologie ’). As for me, I always saw the second pair of 
antennae, even in large specimens of Apus, measuring 3 5 cent, from the head 
to the extremity of the abdomen. The first pair has, in these specimens, a 
