8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



From these early Pre-Cambrian arthropods (fig. i C) in which all 

 the appendages were fully segmented ambulatory legs, the trilobites 

 branched off by specialization of the body structure, but with no es- 

 sential differentiation of the appendages. In the other derivative 

 groups, however, the appendages took on different forms adapting 

 them to various uses, but the number retained for walking is charac- 

 teristic of the several modern arthropod groups. The myriapods use 

 most of their postoral appendages for progression ; the Malacostraca 

 (D) use five or more pairs for walking, except where some of these 

 have been modified for grasping ; Limulus and the arachnids use four 

 pairs, the insects three. That the ambulatory limbs, when limited in 

 number, should in all cases be those of the middle part of the body, 

 though not necessarily the same appendages, ""f ollows from the me- 

 chanical necessity of balance. The anterior appendages become sen- 

 sory and gnathal in function ; those of the abdomen have been modi- 

 fied for various purposes, such as respiration, silk spinning, copulation, 

 egg laying, or swimming. 



The modern arthropods comprise two distinct groups, the Chelicer- 

 ata and the Mandibulata. In the chelicerates the first postoral ap- 

 pendages are a pair of pincerlike chelicerae that serve for feeding, 

 and the ancestors of this group were probably closely related to the 

 ancestors of the trilobites. The principal feeding organs of the 

 mandibulates are a pair of jaws, the mandibles, formed of the second 

 postoral appendages. The Mandibulata, including the crustaceans, 

 the myriapods, and the insects, are certainly a monophyletic group, 

 but their origin and their interrelationships are obscure. 



Among the Crustacea the malacostracan type of organization (fig. 

 I D), in which the thoracic appendages are typically ambulatory and 

 the abdominal appendages natatory, would appear to be more primitive 

 than the entomostracan types because it more closely conforms with 

 the structure of other arthropods, and could be more directly derived 

 from that of a primitive walking arthropod (C). The entomostracan 

 forms, therefore, have been secondarily reconstructed for a purely 

 pelagic life by a readaptation of the thoracic appendages for swimming. 



If we accept the premise that the original arthropod (fig. i C) was 

 a simple animal with jointed legs along the entire length of a uniformly 

 segmented body, the crustaceans were derived from this common 

 arthropod ancestor by specializations that established the generalized 

 crustacean structure (D). Developmental recapitulation of adult 

 crustacean structures, therefore, can go back only to the beginning 

 of adult crustacean evolution. The embryo, however, starts its de- 

 velopment from a single cell and the free larva completes development 



