Page:The Eurypterida of New York Volume 1.pdf/151

 with that of the merostomes, but there appears to exist from the beginning a striking difference in the strong differentiation of the preabdomen and the postabdomen early in the embryonic life of the scorpion, while in the eurypterids that difference between the two parts of the abdomen is still obscure in the larva, as it is in Limulus. In the scorpion the preabdomen is broad and bulky while the postabdomen or tail is abruptly set off, very narrow and flexed upon the ventral surface [see text fig. 30].

The abdominal appendages of the embryo of the scorpion [see text fig. 29] atrophy except for the first pair which form the "combs." In the places of the others the lungbooks appear, resulting from paired invaginations, the walls of which subsequently become plicated. In the merostomes, on the other hand, the preabdominal appendages remain throughout life and bear the branchial lamellae. It is the current opinion of zoologists that the lungbooks of the scorpion are derived from such branchiate abdominal appendages as those of Limulus, the conversion of one set of organs into the other being supposed to have been effected by the formation, behind each pair of abdominal appendages, of an invagination which, deepening, has carried in with it the branchial lamellae. Brauer [1895, p. 373] has recognized the origin of the lungbooks from gills.

In summing up the comparison between the embryonic and larval stages of the scorpion and the larvae of the eurypterids we may say that, (1) the general homologies of the two are very apparent in the composition of the carapace and abdomen of an equal number of segments, but that, (2) while in the scorpion the segmentation is completed long before the hatching, in the eurypterids the larvae in the nepionic stage still lack the full complement of segments, recalling the trilobites in this feature and clearly representing a more primitive condition; (3) there are a number of distinct differences in the larvae of the eurypterids and of the scorpions,