słonie: Hagfish

Evidence: 100 • authority: 72

Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped jawless fish (occasionally called slime eels). Hagfish are the only known living animals that have a skull but no vertebral column; however, they do have rudimentary vertebrae. Hagfish are marine predators and scavengers that can defend themselves against other larger predators by releasing copious amounts of slime from mucous glands in their skin. Although their exact relationship to the only other living group of jawless fish, the lampreys, was initially the subject of controversy, genetic evidence suggests that hagfish and lampreys are more closely related to each other than to jawed vertebra…

Evidence: 68 • authority: 72

Fish began evolving about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages were jawless. Early examples include Haikouichthys. During the late Cambrian, eel-like jawless fish called the conodonts, and small mostly armoured fish known as "ostracoderms", first appeared. Most jawless fish are now extinct; but the extant lampreys may approximate ancient pre-jawed fish. Lampreys belong to the Cyclostomata, which includes the extant hagfish, and this group may have split early on from other agnathans. The earliest jawed vertebra…

Evidence: 61 • authority: 72

Vertebrates (), also called craniates, are animals with a vertebral column and a cranium. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebrata ( VUR-tə-BRAY-tə) with some 65,000 species, by far the largest ranked grouping in the phylum Chordata. The vertebrates include mammals, birds, amphibians, and various classes of fishes and reptiles. The two major living vertebrate subgroups are the jawless Cyclostomi (including hagfish and lampreys), and the jawed Gnathostomata. The jawed fish include both the cartilaginous fish and the bony fish. Bony fish include the lobe-finned fish, which gave rise to t…

Evidence: 54 • authority: 72

Fish anatomy is the study of the form or morphology of fish. It can be contrasted with fish physiology, which is the study of how the component parts of fish function together in the living fish. 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, as might be observed on a dissecting table or under a microscope, and the latter dealing with how those components function together in living fish. The anatomy of fish is often shaped by the physical characteristics of water, the medium in which fish live. Water is much denser than air, holds a relatively small amount of…

Evidence: 47 • authority: 72

A chordate ( KOR-dayt) is a bilaterian animal belonging to the phylum Chordata ( kor-DAY-tə). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other taxa: a notochord, a hollow dorsal nerve cord, an endostyle or thyroid, pharyngeal slits, and a post-anal tail. In addition to the morphological characteristics used to define chordates, analysis of genome sequences has identified two conserved signature indels (CSIs) in their proteins: cyclophilin-like protein and inner mitochondrial membrane protease ATP23, which are exclusively shared by all vertebrates, tunicates, and cephalochorda…

Evidence: 39 • authority: 88

Vertebrate features revealed in the rudimentary eye of the Pacific hagfish ( <i>Eptatretus stoutii</i> ). SUMMARY Hagfish eyes are markedly basic compared to the eyes of other vertebrates, lacking a pigmented epithelium, a lens, and a retinal architecture built of three cell layers – the photoreceptors, interneurons & ganglion cells. Concomitant with hagfish belonging to the earliest-branching vertebrate group (the jawless Agnathans), this lack of derived characters has prompted competing interpretations that hagfish eyes represent either a transitional form in the early evolution of vertebrate vision, or a regression from a previously elaborate organ. Here we show the hagfish retina is not…