ARACHNIDA. 



THE Arachnida may be distinguished from the Hexapoda (Insects), 

 Chilopoda (Centipedes), and -Diplopoda (Millipedes) by the entire 

 absence of the pair of feeler-like appendages, known as antennae, 

 which are affixed to the front of the head in these three classes of 

 animals, and also by the fact that there is no differentiated head 

 furnished with two or three pairs of appendages modified so as to 

 act solely as jaws. la the remaining great class of Arthropoda 

 the Crustacea, which possesses numerous terrestrial species, there 

 are two pairs of antenniform appendages at the anterior end of 

 the body and at least three pairs of appendages modified as jaws. 



The Arachnida may be characterized in detail as follows * : 



Arthropodous animals in which the body in the adult is never 

 composed of more than eighteen segments (somites) and is divisible 

 into two main regions an anterior or cephalothorax, and a posterior 

 or abdomen. 



The cephalothorax consists of six somites, each of which is pro- 

 vided with a pair of appendages. The somites are usually welded 

 together and covered above by a dorsal shield or carapace rarely 

 the posterior two somites remain distinct from the others, and are 

 furnished with one or two dorsal plates, separate from the main 

 portion of the carapace. The carapace is then said to be segmented. 

 The ventral surface of the cephalothorax is typically supplied with 

 one or more median plates (sterna) ; sometimes the sterna are 

 partially or wholly obliterated by the ingrowth of the basal 

 segments of the appendages, which meet in the median ventral 

 line. Sometimes the sternal plates are united, and form a single 

 ventral plate to the cephalothorax. 



Eyes. Except in some degenerate forms, the fore part of the 

 carapace, sometimes called the cephalic or head-region, is furnished 

 with simple eyes, which in undegenerate species are usually eight 

 in number, two being situated close together in the middle line 

 (median eyes), and three on each side (lateral eyes), set in a cluster 

 or separated from each other. 



Mouth. The mouth is a minute aperture placed near the lower 

 part of the anterior extremity of the cephalothorax. It is bordered 

 above by a membranous or horny upper lip (labrum), and usually 

 below by the anterior sternal plate which acts as a lower lip or 

 labium, 



* Some of the degenerate Mites and Ticks furnish exceptions to many of the 

 characters contained in this definition. 



