Answer: If evidence from fossil and living forms is combined, the Agnatha are distinguishable from the other craniates (Gnathostomata) by what they lack: jaws, lateral fins supported by fin rays, vertebrae, a horizontal semicircular canal in the ear, and genital ducts. The nervous, sensory, endocrine, circulatory, excretory, and muscular systems have the same basic structure as those of gnathostomes but are generally simpler. Unlike the slitlike gills of gnathostomes, the gills of agnathans are pouchlike and open through pores. The presence in the ammocoete larva of an endostyle, a gland that otherwise is found only in protochordates (such as the amphioxi and tunicates), suggests that the Agnatha represent an evolutionary level intermediate between the protochordates and gnathostomes; however, the degree of specialization of known agnathans (particularly the single nostril of most forms) rules out the possibility that they are ancestral to the gnathostomes.
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