ARTHROPODA — ARACHNIDA 3 1 1 



That trilobites may possibly be distantly related to these 

 is suggested by the three-lobed structure of the dorsal shield 

 of many of the Merostomata. This trilobation as well as the 

 separation of the thorax into distinct segments is complete in 

 BeHnurus (Devonian-Pennsylvanian),Neolimulus (Silurian), and 

 Aglaspis (Cambrian), This relationship is further corroborated 

 by the Middle Cambrian Sidneyia of the order Limulava, 

 in which to typical eurypterid affinities are added the trilobite- 

 like antennae and biramous appendages, and the higher 

 crustacean caudal fin. On the other hand, there is nothing to 

 suggest a crustacean nauplius form in the ontogeny of the 

 Eurypterida, Limulus or the scorpions. 



Derivation of name. — Greek arachne, a spider, referring 

 to the typical example of this class. 



The Arachnida are subdivided into sixteen orders. 



Order i, Xiphosura (including Synxiphosura) 



Body trilobed by longitudinal furrows. Cephalothorax 

 covered by a large, semicircular carapace. Abdomen of seven 

 to ten segments including the long, movable spine-like terminal 

 segment, — the telson. In at least some genera there are simple 

 eyes present in addition to the compound eyes. Respiration 

 by lamelliform gills (book-gills) attached to the abdominal 

 appendages. Marine and fresh water dwellers. Known from 

 the Cambrian to the present. 



This order, in some of its genera, resembles the trilobites but 

 differs in the presence (at least in most of the genera) of both 

 simple and compound eyes, in the absence of free cheeks and 

 of an annulated pygidium, and especially in the character of its 

 appendages, such as the presence of book-gills instead of limb 

 fringes for respiration, and six pairs of legs about the mouth 

 instead of a pair to each segment. 



Derivation of name. — Greek xiphos, sword, + oura, tail ; 

 referring to the sword-shaped tail or telson. 



