Derivation <ind llomologioi of some Articulates. 503 



alKlomiual limbs, and the only true feet wliieli tlioy have are 

 also at base mouth-organs, that is organs that pertain to the 

 liea<l. Moreover, as has been shown by Packard and others 

 for the Limuhis, they do not pass through the Nauplius stage 

 in their development. These diversities and agreiMnenfcs 

 appear to indieate a derivation for the Limuloids nearly like 

 that of the Crustacean type, Ijut jirobably not from Crusta- 

 ceans. But since Limuloids cannot yet be proved to have 

 existed before the Trenton period in tlie Lower Silurian, a 

 derivation from some species related to the Ceratiocirids is 

 possible. Since many, if not all, of the Eurypterids were 

 freshwater or brackish-water species, the transfer to fresh 

 water may have been an incident attending the divergence, 

 and also an explanation of their attaining so great dimensions, 

 fresh water having been their protection. The large Eury- 

 pterids, several feet in length, would have been helpless 

 among Sharks and Ganoids. 



Derivation of Arachnids. — The line to the lower and earlier 

 Arachnids, that is, to the Scorpions, leads up, according to 

 Van Beneden, Packard, and others, from the early Plerygntus- 

 like Limuloids. The early Scorpion, as well as the modern 

 kinds, has the same number of body-segments as a Eary- 

 pterii^ or Pterygotus — namely, 7 thoracic and 6 abdominal 

 (precisely the normal number in Crustaceans), — the same 

 cephalic relations of the legs, the same absence of abdominal 

 appendages, a like absence of thoracic appendages from all 

 the segments excepting the first two, and similar functions in 

 the members pertaining to these two segments. Further, 

 according to B. Peach, these early Limuloids sometimes 

 have, like the Scorpions, pairs of " combs " or pectinated 

 organs on the underside of some of the thoracic segments. 



But in this change from an aquatic to a terrestrial species 

 the upward progress in structure was great. The four poste- 

 rior pairs of feet in the terrestrial Scorpion have no longer 

 the low-grade feature of serving as jaws as well as feet, but 

 are simply feet ; they are the chief organs of locomotion, and 

 only those of the anterior pair are appendages to the mouth. 

 The antennaj are shortened to pincers (falces), that also serve 

 the mouth. The four pairs of feet are thus cephalic organs, 

 if comparison be made with the Limuloids and Crustaceans, 

 though in arachnology they are called thoracic. In the later 

 true Spiders the body had lost its true Eurypteroid abdomen, 

 but had still, in Palasozoic species, its distinctly segmented 

 thorax ; and this thorax is the abdomen of araclniology. It 

 is segmented in some modern species, while in others the 

 subdivisions have become obsolete or are but faintly indicated^ 



34* 



