NO. II LEGS OF PRIMITIVE ARTHROPODS EWING 3I 



each provided with a triangular sternite, situated between the subcoxae 

 of these segments. The second and third cephalothoracic segments 

 are without a sternite, the subcoxae of each practically touching on the 

 median line. This condition of the second and third segments is rather 

 remarkable in this group, as the subcoxae of these segments, like 

 those of the first and last, are movable and undergo a rolling motion 

 while the arachnid is walking. 



In tarantulas (Araneida) the movable basal segments of the legs 

 (coxae?) come close together on the median line. The first three 

 pairs are separated by a sternal ridge, but the coxae of the last pair 

 are contiguous. Muscle attachments in the cephalothorax of spiders 

 are largely from an elaborate endosternite, the origin of which in the 

 Solpugida has been worked out by Bernard (1896), who finds that 

 it results from the fusion of infolding apodemes, particularly between 

 the third and fourth segments. 



In the Solpugida the subcoxae meet on the median line, but are 

 not movable. They retain the shape and the function of a basal leg 

 segment. Bernard (1896), as already quoted, believed that the so- 

 called abdominal sternites of solpugids represent the rudiments of 

 a flattened leg. Holding such a view it is interesting to note that 

 in the same paper he describes and figures a hypothetical primitive 

 arachnid as having typically a sternal plate between each pair of 

 coxae. If abdominal sternites in the Solpugida are derived from 

 the appendages why not the thoracic sternites, not possibly in this 

 group but in many other arthropods? 



STERNA IN PAUROPODA 



In Panropus there are no sternites to the leg-bearing segments 

 except for the segment bearing the last pair. This segment has an 

 anterior and posterior plate (pi. 11, fig. 33). From each subcoxa 

 there extends upward along the pleuron a long rod-like apodeme 

 (pi. II, fig. 33). During leg movements the subcoxa rocks, or rolls, 

 along the axis of this apodeme. On the first body segment there is 

 a pair of ventral discs, believed to represent vestigial legs (see 

 page 25), and the last visible body segment is covered below with 

 a very large sternal plate. 



STERNA IN SYMPHYLA 



There is a marked contrast between the Symphyla and the Pauro- 

 poda in regard to the sternal region. In the genera Scutigerella, 

 Symphylella, and Hanseniella, not only is there always a pair of 



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