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Which body structures do agnathans lack compared with gnathostomes?

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lamprey — agnathan Lamprey (*Lampetra*) on rainbow trout.

Agnathans are distinguished from gnathostomes (jawed vertebrates) primarily by the absence of several key anatomical structures. Compared with gnathostomes, agnathans lack the following:

Jaws: Unlike gnathostomes, agnathans do not possess a hinged jaw apparatus.12
Lateral fins: They lack paired lateral fins supported by fin rays.12
Vertebrae: Agnathans do not have a true vertebral column.12
Horizontal semicircular canal: They lack the horizontal semicircular canal found in the inner ear of gnathostomes.12
Genital ducts: Agnathans do not possess genital ducts.12

Additionally, while agnathans share the same basic organ systems as gnathostomes, these systems are generally simpler in structure. Furthermore, their gills are distinctively pouchlike and open through pores, whereas gnathostomes possess slitlike gills.21