Class Agnatha, probably the first class in the subphylum Vertebrata, consists of marine animals apparently similar to fish but with some very noticeable differences. The agnathans lack both jaws and paired fins. Instead, they have a circular toothed outgrowth with which they bore into the side of a fish and suck the blood of its victim. Lampreys, a member of this class, decimated Great Lakes' fish populations when they were introduced in the early 1900's. All agnathans are marine, and the larvae use their gills to trap food particles, similar to the feeding method of the lancelets and tunicates, suggesting that agnathans are the closest vertebrate relatives of the invertebrate chordates. Hinged jaws, seen by all other classes of vertebrates, probably evolved from the skeleton that supports the gill slits. Two pairs of skeletal rods probably evolved to become jaw bones, a process that occurs in fish larvae today.