PALEOZOIC TIME — CARBONIC. 721 



The Nauplius, or larval form, of a Crustacean shows, by its having but 

 three pairs of limbs (two besides an antennary pair), that the type is not 

 successional to a many-jointed Annelid, but rather to some type of Rotifer. 



The Eurypterids, the early form of the Limuloids, are related to Crusta- 

 ceans in the number of body segments, it being 19, as in the Tetradecapods ; 

 aiid 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 to be true in Limulus by Packard and others, they do not 

 pass through the Nauplius stage in their development. These diversities 

 and agreements appear 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 Ceratio- 

 carids is possible. 



Since many of the Eurypterids were fresh-water or brackish-water species, 

 the transfer to fresh water may have been an incident attending the diver- 

 gence ; and also an explanation of their attaining so great dimensions, fresh 

 waters having been their protection. The large Eurypterids, several feet in 

 length, would have been helpless among Sharks and Ganoids. 



8. Myriapods, Arachnids, Insects. — Arachnids and Insects have their 

 Upper Silurian species, but the first of Myriapods yet found are from the 

 Lower Devonian. 



The remains of Insects in the Lower and Upper Silurian, together with 

 those of the Devonian and Carboniferous, indicate, according to Scudder and 

 Brongniart, that Hemipteroid, Neuropteroid, and Orthopteroid species, and 

 more or less intermediate forms, were then the common kinds. Nothing 

 about the earlier forms of Insects is known. The existence of six pairs of 

 wings instead of four, that is, one for each segment of the thorax, may have 

 been a primitive feature; but this is not considered probable. The great 

 size of some of the Devonian and Carboniferous species is a remarkable 

 feature of the age. A spread of wing exceeding two feet is a size now 

 existing only in large Bats and Birds. 



The iS'europteroids and Orthopteroids were the predominant types ; and 

 among them were intermediate species, as has been already illustrated. The 

 latter type as regards the family of Cockroaches, as explained by Scudder, 

 culminated before the close of the Paleozoic. Previous to its close, the wings 

 of the two pairs in these species were alike in diaphaneity, very nearly alike 

 in size, and hence equally efficient as flying organs. But in the following 

 period (as illustrated by specimens from Colorado), the anterior pair begin 

 to show some thickening and obscuration ; and in the present era nearly all 

 the species have the anterior wings coriaceous, and fitted to serve, as in 

 Beetles, almost solely or solely as wing covers. The posterior wings, on 

 Dana's manual — 46 



