NO. II LEGS OF PRIMITIVE ARTHROPODS EWING II 



through a single segment. Although segment seven has no muscles 

 originating in it, it should be regarded as a true segment because the 

 flexor muscle, of the sixth segment inserts on its base and its articula- 

 tion with segment eight indicates that the hinge v^as once functional. 

 The conditions of this segment are entirely analogous to those of the 

 trochanter in a lepismid leg where the muscle is usually lacking. The 

 writer finds the leg segments of Pycnogonida entirely homologous 

 with those of other generalized arachnids and consisting of the fol- 

 lowing: Subcoxa, coxa, trochanter I, trochanter II, femur, patella, 

 tibia, tarsus and pretarsus. 



In the Ricinulei, according to Hansen and S^rensen (1904), the 

 second and third pairs of legs have eight segments. These authors re- 

 gard the basal segment as the coxa and allow for two trochanters, a 

 patella and a metatarsus. Their figures would indicate that the leg of 

 a member of the Ricinulei is entirely of the type found in the solpugids, 

 the primitive phalangids and primitive mites. This being the case, the 

 first segment should be regarded as the subcoxa, the femur would 

 become the trochanter and the other segments would be renamed 

 according to their positions, the term metatarsus being discarded. 



CONCLUSIONS IN REGARD TO ARACHNID LIMBS 



It appears, therefore, that the generalized arachnid limb is com- 

 posed of eight segments in addition to the foot or claws. This is a 

 condition found today in the Solpugida, the Phalangida, the gen- 

 eralized Acarina, the Ricinulei and the Pycnogonida. Such a con- 

 dition gives much weight to Hansen's contention that eight segments 

 should be recognized in the crustacean limb. Since the foot, or claws 

 in Arachnida. as in all arthropod groups, should be considered as a 

 segment, one segment more in this class is to be reckoned with than in 

 Crustacea, Insecta, or apparently in any other arthropod class. This 

 segment should be called the patella. It is situated between the femur 

 and the tibia. 



The subcoxa is evident and well developed in most arachnid groups. 

 The Arachnida thus shows in this respect a condition not found in 

 the Crustacea and a more primitive one. 



The tarsus, although frequently possessing a number of false seg- 

 ments, as in insects, is of a more primitive type than the insect tarsus. 

 This is particularly true in regard to its musculature, there being an 

 extensor of the claws present, while the flexor of the claws sends 

 fibers to attach to the segment proximal to the tibia. 



