MECHANICAL ORIGIN OF THE LIMBS 37 



trilobite stage, where they are more solid, evenly jointed, but not end- 

 ing in claws; and by their comparatively great numbers (as in the 

 trilobite, Triarthrus) fully supporting the body on the bottom of the 

 sea. In Limulus they are much fewer in number, thicker, and acting 

 as firm supports, the cephalic limbs of use in creeping, and ending in 

 solid claws. 3, The third stage is the long slender swimming head- 

 appendages of the nauplius stage of Crustacea. 



As regards the evolution of limbs of terrestrial arthropods, we 

 have the following stages : 1, the soft unjointed limbs of Tardi- 

 grades, ending in two claws, and those of Peripatus, and the pseudo- 

 or prop-legs of certain dipterous larvae; 2, finally the evolution of 

 the long, solid, jointed limbs of Pauropus and other primitive myrio- 

 pods, the legs forming solid, firm supports elevating the body, and 

 enabling the insect to drag itself over the ground or to walk or run. 

 When the body is elongated and many-segmented, the legs are neces- 

 sarily numerous ; but when it is short, the legs become few in number, 

 i.e. six, in the hexapodous young of myriopods and in insects, or 

 eight in Arachnida. Whenever the legs are used for walking, i.e. 

 to raise and support the body, they end in a solid point or in a pair 

 of forceps or claws. On the other hand, as in phyllopods, where 

 the legs are used mainly f.or swimming, they are unarmed and are 

 soft and membranous, or, as in the limbs of the nauplius or zoea 

 stage of crustaceans, end in a simple soft point, which often bears 

 tactile setae. 



The tarsal joints are more numerous in order to give greater flexi- 

 bility to the limb in seizing and grasping objects, both to drag the 

 body forwards and to support it. 



Unlike those of the Crustacea, the limbs of insects are not primi- 

 tively biramose, but single, the three-lobed first maxillae, and sec- 

 ondarily bilobed second maxillae being the result of adaptation. 

 Embryology on the whole proves the truth of this assumption ; the 

 maxillae of both pairs are at first single buds, afterwards becoming 

 lobed. All the appendages of the body, including the ovipositor or 

 sting, are modified limbs, as shown by their embryological develop- 

 ment. 



It is noticeable that in the crab, where the body is raised by the 

 limbs above the bottom, it is much shorter and more cephalized than 

 in the shrimps. Also in the simply walking and running spiders, 

 the hind-body is shorter than in scorpions, while in the running and 

 flying insects, such as the Cicindelidae, and in the swiftly flying 

 flies and bees, there is a tendency to a shortening of the body, 

 especially of the abdomen. The long body of the dragon-fly is an 



