limulids, and may here briefly enumerate them. The nepionic stages of both have in common:
- The large size of the carapace
- Its broad border
- Its distinct median, glabellalike ridge (best seen in Stylonurus myops)
- The smaller number of body segments (nine or less, seen also in S. myops)
- The lack of differentiation of the segments
- The undeveloped telson
As differences appear:
- The terete or conical abdomen in the young eurypterids in contrast with the broadened abdomen of Limulus, and
- Large larval eyes in the eurypterids
We consider both these differential characters as due to purely adaptive changes. The broader abdomen of the larvae of Limulus results from the earlier appearance of the broad abdomen of the mature type through acceleration, and the adaptive nature of the large larval eyes has been fully discussed on page 119. For the reasons here given we find ourselves in agreement with those authors who have united the eurypterids and limulids under Dana's subclass Merostomata.
If the relationship of the eurypterids with the king crabs is so close that it places them in one subclass, the eurypterids will have to follow the Limulidae in their wanderings in the zoological system.
The limulids were, as is well known, classed with the crustaceans, chiefly on account of their aquatic habit and branchial respiration, since the Arthropod phylum was by common consent divided into two subphyla, the Branchiata and Tracheata. Opposition arose to this classification through the recognition of the affinities of Limulus with the arachnids, first suggested by Strauss-Durkheim, and especially elaborated by Van Beneden, Lankester, Kingsley and Laurie, though not without opposition from such authorities as Packard, Woodward, Thorell and Lindström,
