It is also found on the carapace of crabs and limulus, and on mollusks.
Of all these the only member alive at the present time is limulus, or the King-Crab.
limulus has two pairs of these, Scorpio as many as six pairs.
I do not know how far this system is represented in limulus or Scorpio.
Fortunately, there is still alive on the earth one member of this remarkable group—the limulus, or King-Crab.
See Patten for important observations on the neuromeres, &c., of limulus and Scorpio.
Patten considers the Pteraspids as derived from primitive arachnid or spider-like forms having a bony carapace as limulus has.
Numerous rhabdomeres (grouped as rhabdoms in limulus) are found in the retinal layer of the central eyes also.
Linnus thought that the fossil trilobites were closely allied to the limulus.
Thus an organ newly discovered in Scorpio was found to have its counterpart in limulus.