Superclass AGNATHA
Cyclostomes
(Cyclostomata)
Cyclostomes
32
The agnathans are an ancient group of chordates that first appear in fossils of
the Middle Ordovician period. They are characterised by the absence of true
jaws derived from gill arches. They also lack paired fins (pectoral fins are
developed in some), true teeth and vertebrae; the gills are carried on the inner
edge of the gill arches, which are fused to the neurocranium. The internal
skeleton is entirely cartilaginous, but many of the fossil agnathans had a welldeveloped armour of bony plates, especially about the head region.
Two classes are recognised: the Cephalaspidomorphi and the Pteraspidomorphi. The cephalaspid ancestors are supposed to have given rise to the
freshwater lampreys, and the marine hagfishes are thought to be descendants
of the early pteraspids (Jarvik, 1980). As we are here concerned only with the
modern hagfishes, the fossil pteraspids are not considered further.
Cyclostomes
(Cyclostomata)
Cyclostomes
32
The agnathans are an ancient group of chordates that first appear in fossils of
the Middle Ordovician period. They are characterised by the absence of true
jaws derived from gill arches. They also lack paired fins (pectoral fins are
developed in some), true teeth and vertebrae; the gills are carried on the inner
edge of the gill arches, which are fused to the neurocranium. The internal
skeleton is entirely cartilaginous, but many of the fossil agnathans had a welldeveloped armour of bony plates, especially about the head region.
Two classes are recognised: the Cephalaspidomorphi and the Pteraspidomorphi. The cephalaspid ancestors are supposed to have given rise to the
freshwater lampreys, and the marine hagfishes are thought to be descendants
of the early pteraspids (Jarvik, 1980). As we are here concerned only with the
modern hagfishes, the fossil pteraspids are not considered further.