ARTHROPOD A — ARACHNID A 313 



the first pair probably always chelate, and the last pair, the 

 broad, powerful paddle, used in swimming and in burying itself 

 in the mud. The anterior six abdominal segments consist of 

 the anterior genital operculum and the posterior five pairs of 

 broad, leaf-like appendages, probably respiratory; these six 

 are apparently comparable to the operculum and book-gills 

 of Limulus (see relationship of the Arachnida, p. 309). The 

 mode of Hfe was varied; it included swimming, crawUng and 

 burrowing. 



Though the order is entirely extinct, yet the growth stages 

 of many of the Eurypterida are well known. In the Shawan- 

 gunk grit (Middle Silurian) of southeastern New York excel- 

 lent series have been obtained, some of the stages measuring 

 less than two millimeters in length. The larval stages have 

 a comparatively larger carapace, with compound eyes larger 

 and nearer the margin, swimming legs usually larger, and a 

 less number of segments to the body. The later larval stages 

 of the mid-Paleozoic species show some characters present in the 

 adult Cambrian and Ordovician forms. 



The Eurypterida (Cambrian to Permian) were probably at 

 first marine, later (Devonian to Pennsylvanian) they became 

 adapted to brackish water, and finally (Permian) to a fresh 

 water environment. The earlier species are usually found 

 associated with the marine cephalopods and trilobites, while 

 the later forms occur in strata along with land plants, insects, 

 scorpions and fresh water Amphibia. (There are some students 

 who believe that all eurypterids from the Cambrian to the 

 Permian w^re fresh water inhabitants, because of the apparent 

 evidence that the inclosing sediments were deposited upon land 

 and not in the sea.) Some forms, as Pterygotus, attained at 

 times a length of nine feet. Stylonurus is another large form 

 from the Silurian and Devonian, distinguished by the possession 

 of two pairs of enormously elongated appendages. 



Derivation of name. — Greek eurys, broad, + pteron, wing, 

 referring to the broad, wing-like pair of legs. 



