270 ARACHNIDA—XIPHOSURA CHAP. 
bearing appendages can be flapped to and fro, and they seem to 
be at times held apart by the flabellwm, a spatulate process which 
Patten and Redenbaugh regard 
as a development of the median 
sensory knob on the outer side 
of the coxopodite of the last 
pair of walking limbs. 
Limulus has no trace of 
Malpighian tubules, structures 
which seem often to develop 
only when animals cease to 
live in water and come to live 
in air. The Xiphosura have 
retained as organs of nitrogen- 
ous excretion the more primi- 
tive nephridia, or coxal glands 
as they are called, im the 
Arachnida. They are _ red- 
brick in colour, and consist of 
a longitudinal portion on each 
side of the body, which gives 
Fia. 
155. — Diagram 
Limulus, from the posterior side, show- 
of the first gill of 
ing the distribution of the gill-nerve to 
the gill-book (about natural size). After 
Patten and Redenbaugh. 1, Inner lobe 
of the appendage; 2, outer lobe of 
appendage ; 3, median lobe of appendage ; 
4, gill-book: 5, neural nerve of the ninth 
neuromere ; 6, internal branchial nerve ; 
7, gill-nerve ; 8, median branchial nerve ; 
9, external branchial nerve. 
off a lobe opposite the base of 
the pedipalps and each of the 
first three walking-legs—in 
the embryo also of the cheli- 
cerae and last walking legs, but 
these latter disappear during 
development. A duct leads 
from the interior of the gland and opens upon the posterior 
face of the last pair of alin legs but one. 
The nervous system has been very fully described by Panton 
and Redenbaugh, and its complex nature plays a large part in the 
ingenious speculations of Dr. Gaskell as to the origin of Verte- 
brates. It consists of a stout ring surrounding the oesophagus 
and a ventral nerve-cord, composed—if we omit the so-called 
fore-brain—of sixteen neuromeres. The fore-brain supplies 
the median and the lateral eyes, and gives off a median nerve 
which runs to an organ, described as olfactory by Patten, situated 
in front of the chelicerae on the ventral face of the carapace. 
Patten distinguishes behind the fore-brain a mid-brain, which 
