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Most male fish have two testes of similar size. In the case of sharks, the testes on the right side is usually larger [citation needed].The primitive jawless fish have only a single testis, located in the midline of the body, although even this forms from the fusion of paired structures in the embryo.
Agnatha (/ ˈ æ ɡ n ə θ ə, æ ɡ ˈ n eɪ θ ə /; [3] from Ancient Greek ἀ-(a-) 'without' and γνάθος (gnáthos) 'jaws') is a paraphyletic infraphylum [4] of non-gnathostome vertebrates, or jawless fish, in the phylum Chordata, subphylum Vertebrata, consisting of both living (cyclostomes) and extinct (conodonts, anaspids, and ostracoderms, among others).
Thelodonti ('feeble-teeth') are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armour. There is much debate over whether the group of Palaeozoic fish known as the Thelodonti (formerly coelolepids [11]) represent a monophyletic grouping, or disparate stem groups to the major lines of jawless and jawed ...
Hagfish, of the class Myxini / m ɪ k ˈ s aɪ n aɪ / (also known as Hyperotreti) and order Myxiniformes / m ɪ k ˈ s ɪ n ɪ f ɔːr m iː z /, are eel-shaped jawless fish (occasionally called slime eels). Hagfish are the only known living animals that have a skull but no vertebral column, although they do have rudimentary vertebrae. [3]
Jawless fish belong to the superclass Agnatha in the phylum Chordata, subphylum Vertebrata. Agnatha means 'un-jawed, without jaws' (from Ancient Greek). [12] It excludes all vertebrates with jaws, known as gnathostomes. Although a minor element of modern marine fauna, jawless fish were prominent among the early fish in the early Paleozoic.
In those days, placoderms were thought to be shelled jawless fish akin to ostracoderms. Some naturalists even suggested that they were shelled invertebrates or even turtle -like vertebrates. In the late 1920s, Dr. Erik Stensiö , at the Swedish Museum of Natural History in Stockholm , established the details of placoderm anatomy and identified ...
Jawless fishes from the Ordovician Period — 488.3 million to 443.7 million years ago — are called ostracoderms, after their armored skin, and most of them are known from fossils that only ...
In practice, fish anatomy and fish physiology complement each other, the former dealing with the structure of a fish, its organs or component parts and how they are put together, such as might be observed on the dissecting table or under the microscope, and the latter dealing with how those components function together in living fish. The ...