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cephalaspidomorphi
Cephalaspidomorphs are a group of jawless fishes named for the cephalaspids, a group of osteostracans. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day.
==Biology and Morphology==
Cephalaspidomorphi were like most contemporary fishes very well armoured. The head shield was particularly well developed, protecting the head, gills and the anterior section of innards. The body was in most forms well armoured too. The head shield had a series of grooves over the whole surface forming an extensive lateral line organ. The eyes were rather small and placed on the top of the head. There was no jaw proper. The mouth opening was surrounded by small plates making the lips flexible, but without any ability to bite.
No internal skeleton is known outside of the head shield. If they had a vertebral column at all, it would have been cartilage rather than bone. Likely, the axial skeleton consisted of an unsegmented notochord. A fleshy appendage emerged laterally on each side behind the head shield, functioning as pectoral fins. The tail had a single, wrap-around tail-fin. Modern fishes with such a tail are rarely quick swimmers, and the cephalaspidomorphi were hardly very active animals. They likely spent much of their time semi-submerged in the mud. They also lacked a swim bladder and would not have been able to keep afloat without actively swimming. The head shield provided some lift though, and would have made the cephalaspidomorphi better swimmers than most of their contemporaries.〔 The whole group were likely algae- or filter-feeders, combing the bottom for small animals much like the modern armoured bottom feeders, such as Loricariidae or ''Hoplosternum'' catfish.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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