2 VltACHNIDA. 



Appendages. — The appendages forming the first pair (mandibles, 

 chelic&rce) are situated close together beneath the front edge of the 

 carapace above the mouth. Each consists of two, rarely of three, 

 segments, and is very frequently pincer-like or chelate, the terminal 

 segment (movable finger or digit) closing against a prolongation of 

 the penultimate segment. This prolongation is called the im- 

 movable finger. In other cases the terminal segment closes against 

 the penultimate segment without the interposition of a finger-like 

 prolongation. The appendages forming the second pair (the 

 palpi or chelce) are usually large, but are very variable in function 

 and form, being sometimes prehensile and pincer-like, when they 

 are termed chela?, sometimes tactile and leg-like, when they are 

 termed palpi. Each typically and almost invariably consists of 

 six segments, which, although strictly homologous throughout 

 the class, have unfortunately in descriptive works received different 

 names in different orders, largely on account of the failure of 

 authors to recognize this homology. The basal segment is termed 

 the coxa, or, when it acts as a masticator of food, the maxilla. It 

 is often furnished with a process, the maxillary process, which pro- 

 jects forwards beneath or on one side of the mouth. The second 

 segment is termed the trochanter ; the third the femur or humerus; 

 the fourth the tibia or brachium when the limb is chelate, or patella 

 when it is pediform ; the fifth and sixth are termed respectively 

 hand and movable finger in the former case, tibia and tarsus in the 

 latter. To the tip of the sixth is typically affixed a claw, which 

 may be either freely movable or fused to the segment and scarcely 

 distinguishable from it. The remaining four pairs of appendages, 

 spoken of collectively as the legs, are generally similar or subsimilar 

 in form and subserve locomotion. The first of these four pairs, 

 however, sometimes acts as a tactile organ, and is not used for 

 progression but is carried raised from the ground. Typically these 

 limbs consist each of seven segments, named from base to apex as 

 follows : coxa, trochanter, femur, patella, tibia, protarsus, tarsus. 

 The tarsus is tipped with two or three claws, rarely with one, and 

 may be secondarily subdivided into two or more segments. 



Abdomen. — The abdomen is generally without appendages. 

 When present they are of small size, are set apart for purposes of 

 reproduction and silk-spinning, and are never used for locomotion. 

 The abdomen is either distinctly segmented, being furnished above 

 with a series of dorsal plates (terga) and below with a correspond- 

 ing series of ventral plates (sterna); or no such plates are developed, 

 and the visible external signs of segmentation are either wanting 

 or obscure. At most there are twelve distinct somites in this 

 region, though sometimes there is a skeletal piece movably articu- 

 lated to the last, above the anal aperture. When this structure is 

 present, the last segment and two or more of those that precede it 

 are narrowed and form with the postanal sclerite the so-called 

 tail. Both terga and sterna of the abdomen are generally pitted 

 with a pair of scars (ttigil/a or impressions) which mark the point 

 of attachment of a series of internal dorso-ventral muscles. 



