ARTHROPODA 179 



numbered from the head backwards (I. IV.), and their joints from 

 the base to the free end (1 6). There is also a nomenclature of the 

 leg-joints, thus : coxa, trochanter, femur, tibia, protarsus and tarsus. 

 At the proximal end of the 3rd and 6th joints, a supernumerary 

 segment is more or less clearly defined, and for this reason many 

 authors reckon eight joints to each limb. The tarsus bears a process 

 which is prolonged into two claws, from the distal end of which hangs 

 a membranous structure, the pulvillum. The hip segments are not 

 attached to the body by means of joints, but are anatomically a part 

 of it. Those which bear the first pair of legs are furnished with a 

 stout thorn, directed backwards, while the tarsi of this pair have 

 a small sense-organ upon the dorsal side. 



FIG. 90. Ixodes ricinus (L.). Female. Capitulum with mouth-parts, seen from the 

 ventral surface. 57 : 1. (After a drawing by A. Dampf.) 



The mouth-parts are twofold, consisting of the hypostome and 

 the chelicerse. At the sides of the capitulum, externally to the 

 chelicerse, are the palps. These have four articulations and are 

 characterized by the length of the second and third segments, which 

 are hollowed out upon the ventral surface to form a sort of gutter. 

 The first, or basement joint, is short and the terminal joint is even 

 shorter. The latter proceeds from a terminal hollow in the third 

 joint and is placed at an angle with it, the free end being directed 

 ventrally ; it bears about fifteen small bristles. Between the palps, 

 in the median line, is the rostrum, the most conspicuous feature of 

 which is the hypostome. This is an apparently unpaired organ, 

 placed upon the ventral surface of the rostrum and recognized by 

 many authors as the rostrum itself. Its ventral surface is covered 



