Evolutionary OriginsAgnatha are distinguishable from the other craniates, or Gnathostomata, by what they lack: jaws, paired fins, vertebrae, a horizontal semicircular ear canal, and genital ducts. The nervous system, sensory system, endocrine system, circulatory system, excretory system, and muscular system have the same basic structure as those of gnathostomes but are simpler. Unlike the slitlike gills of the 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 suggests that the Agnatha represent an evolutionary level between the protochordates and the gnathostomes; however, the known specializations of agnathans, such as the single nostril of most forms, rules out the possibility that they are ancestral to the gnathostomes.
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CharacteristicsIn Agnatha, or Jawless Fish, the most common characteristic between them is just that, the absence of their Jaw. However those are not the only similarities between them. They also have:
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References:
Agnatha. (n.d.). Retrieved June 3, 2015, from http://www.fernbank.edu/STT/VertBio/agnatha.htm
Agnathan | fish. (n.d.). Retrieved June 3, 2015, from http://www.britannica.com/EBchecked/topic/9280/agnathan
Agnatha. (n.d.). Retrieved June 3, 2015, from http://www.fernbank.edu/STT/VertBio/agnatha.htm
Agnathan | fish. (n.d.). Retrieved June 3, 2015, from http://www.britannica.com/EBchecked/topic/9280/agnathan