sentinel crab
Webster's Dictionary (GCIDE)
Sentinel \Sen"ti*nel\, n. [F. sentinelle (cf. It. sentinella); probably originally, a litle path, the sentinel's beat,, and a dim. of a word meaning, path; cf. F. sente path. L. semita; and OF. sentine, sentele, senteret, diminutive words. Cf. Sentry.]
1.One who watches or guards; specifically (Mil.), a soldier set to guard an army, camp, or other place, from surprise, to observe the approach of danger, and give notice of it; a sentry. [1913 Webster]
The sentinels who paced the ramparts.— Macaulay.[1913 Webster]
2.Watch; guard. [Obs.] "That princes do keep due sentinel." --Bacon. [1913 Webster]
3.(Zool.) A marine crab (Podophthalmus vigil) native of the Indian Ocean, remarkable for the great length of its eyestalks; -- called also sentinel crab. [1913 Webster]