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Lacking paired fins, [20] adult lampreys have one nostril atop the head [21] and seven gill pores on each side of the head. [12] The brain of the lamprey is divided into the telencephalon, diencephalon, midbrain, cerebellum, and medulla. [22]
Because of shared features such as paired fins, the origins of the jawed vertebrates may lie close to Cephalaspidomorphi. Many biologists no longer use the name Cephalaspidomorphi because relations among Osteostraci and Anaspida are unclear, and the affinities of the lampreys are also contested.
Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly hypocercal tail. They first appeared in the Early Silurian, and flourished until the Late Devonian extinction, [17] where most species, save for lampreys, became extinct due to the environmental upheaval during that time.
A craniate is a member of the Craniata (sometimes called the Craniota), a proposed clade of chordate animals with a skull of hard bone or cartilage.Living representatives are the Myxini (hagfishes), Hyperoartia (including lampreys), and the much more numerous Gnathostomata (jawed vertebrates).
Adult river lampreys measure from 25 to 40 cm (10 to 16 in) for the sea-going forms and up to 28 cm (11 in) for the lake forms. The very elongate body is a uniform dark grey above, lightening to yellowish off-white on the sides and pure white below. Like all lampreys, these fish lack paired fins and possess a circular sucking disc instead of jaws.
The jawless fish (hagfish and lamprey) do not have appendage-like fins. They may not have lost them, but rather, simply retained the form that vertebrates had before the evolution of limbs. [1] There are also a number of fish with elongated bodies that have no fins or reduced appendage-like fins, for example eels and swamp eels. [2]
As embryos, vertebrates still have a notochord; as adults, all but the jawless fishes have a vertebral column, made of bone or cartilage, instead. [citation needed] Vertebrate embryos have pharyngeal arches; in adult fish, these support the gills, while in adult tetrapods they develop into other structures. [9] [10]
Though common and diverse during the Silurian and Devonian, jawless fish are today represented only by lampreys and hagfish, both groups being quite specialized.Lampreys have seven gill pouches (whereas jawed fish have only five), no paired fins, and a rudimentary skeleton of cartilage.