270 



ARACHNIDA XIPHOSURA 



bearing appendages can be flapped to and fro, and they seem to 

 be at times held apart by the fiabellum, a spatulate process which 



Patten and Eedenbaugh 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, in the 

 Arachnida. They are red- 

 brick in colour, and consist of 

 a longitudinal poition on each 



4- 



the gill-book (about natural size). After 

 Patteu aud 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. 



Fig. 155. — Diagram of the first gill of gj^Jg of ^\^q hodiX, which gives 

 Limulus, from the posterior side, show- • ^ ^ c 



ing the distribution of the gill-nerve to off a lobe OppOSltC the base ot 



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 walking legs but one. 



The nervous system has lieen very fully described by Patten 

 and Ptedenbaugh, 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-hrain supplies 

 the median and the lateral eyes, and gives oft' 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-brai7i, which 



