THE EURYPTERIDA OF NEW YORK I45 
with that of the merostomes, but there appears to exist from the begin- 
ning a striking difference in the strong differentiation of the preab- 
domen 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 
. . Figure 30 Later embry- 
as those of Limulus, the conversion of one set of organs onic stage in develop- 
into the other being supposed to have been effected mi view ok chelicerae 
by the formation, behind each pair of abdominal ap-  ?'-?*, walking legs; pe, 
ee j j pecten; st, stigmata ; ab, 
pendages, of an invagination which, deepening, has _ postabdomen. (From 
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, 
