Sea spiders as large as a foot across have been seen crawling along the deep ocean floor from the windows of submersible research vessels. Most of them, however, including those in a Harvard study, are a scant millimeter (.04 inch) in size. But big or small, they boast long snouts, on either side of which grow pincerlike claws.

Zoologists classify them as arthropods, a group that includes all the insects, land-loving spiders, and crustaceans from flea-size shrimp to lobsters. Together they make up the largest class of animals on Earth.

“In all other arthropods, the front section of the brain bears only eyes,” notes Amy Maxmen, 27, a Harvard graduate student who studies sea spiders for her Ph.D. thesis. “Our observation is the first ever of a clawlike appendage arising from that part of the brain. The finding supports assumptions by others that some ancestors of living arthropods once had a pair of pincers or antennae, along with their eyes, extending from the forward parts of their brains.”

“At first sight, this is a rather esoteric finding,” according to two scientists who commented on the discovery as it is reported in the Oct. 20, 2005 issue of the science journal Nature. “But if it is correct, it will shake up the field of arthropod evolution.”

“The evolutionary status of the front part of the brain has been a point of contention for a long time,” Maxmen notes. Prior to her study, no living arthropods had anything but eyes growing out of this part of the head. However, it appeared that some extinct arthropods did. Thus, sea spiders, rather than being odd, water- living relatives of ordinary house spiders, may be the only surviving relatives of arthropods who walked the oceans’ floors some 500 million years ago with appendages attached to the front of their heads.