noun, plural: agnathans
Any of the jawless fish species belonging to the taxonomic superclass Agnatha of the chordates
The agnathans are fish species belonging to the taxonomic superclass Agnatha of the phylum Chordata (chordates) and includes groups of jawless fish. Chordates are primarily characterized by having a notochord, dorsal hollow nerve cord, muscular post anal tail, and pharyngeal slits for at least some period of the life cycle of an animal. The phylum Chordata has three subphyla: Vertebrata (vertebrates), Tunicata (tunicates), and Cephalochordata (cephalochordates). The vertebrates include various animal groups such as fish, amphibians, reptiles, birds, and mammals. A fish pertains to any gill-bearing animals lacking limbs with digits. There are three superclasses of fish species: Agnatha (jawless fish), Chondrichthyes (cartilaginous fish), and Osteichthyes (bony fish).
Apart from having no jaws, extant agnathans are characterized by lacking paired appendages. Most of them do have a caudal fin. Fish species included in this group include the cyclostomes (e.g. lampreys and hagfish) and the extinct ostracoderms (e.g. Pteraspidomorphi, Thelodonti, Anaspida, and Cephalaspidomorphi).
Word origin: Greek khóndros (“grain, corn, cartilage”) + ikhthús (“fish”)
- Kingdom: Animalia
- Phylum: Chordata
- Subphylum: Vertebrata
- Superclass: Agnatha [Cope, 1889]
Other common name(s):
- jawless fish