326 J. D. Dana — Observations on the Derivation 



The Eurypterids, the early form of the Limuloicls, are re- 

 lated to Crustaceans in number of body-segments, it being 19, 

 as in the Tetradecapods ; and in the fact that 13 of these 19 

 segments pertain to the thorax and abdomen. But the wide 

 distinction exists that the Eurypterids have no thoracic or 

 abdominal limbs, and the only true feet which they have are 

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

 head. Moreover, as has been shown, by Packard and others 

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

 in their development. These diversities and agreements ap- 

 pear to indicate a derivation for the Limuloids nearly like 

 that of the Crustacean type, but probably not from Crustaceans. 

 But since Limuloids cannot yet be proved to have existed 

 before the Trenton period in the Lower Silurian, a derivation 

 from some species related to the Ceratiocarids is possible. Since 

 many if not all of the Eurypterids were freshwater or brack- 

 ish-water species, the transfer to freshwater may have been 

 an incident attending the divergence ; and also an explanation 

 of their attaining so great dimensions, freshwater having 

 been their protection. The large Eurypterids, 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 Yan 

 Beneden, Packard and others, from the early Pterygotus-like 

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

 has the same number of body-segments as a Eurypterus or 

 Pterygotus, namely : 7 thoracic, and 6 abdominal (precisely 

 the normal number in Crustaceans) ; the same cephalic rela- 

 tions 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 Scor- 

 pions, pairs of " combs " or pectinated organs on the under- 

 side 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 4 posterior 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 an- 

 tennas 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 Eurypteroid abdomen, but 

 had still, in Paleozoic species, its distinctly segmented thorax ; 



