Craniates - Crainata - the Animal Encyclopedia

Craniates (Craniata) are a group of chordates that includes hagfish, lampreys, and jawed vertebrates such as amphibians, birds, reptiles, mammals, and fishes. Craniates are best described as chordates that have a braincase (also called a cranium or a skull), mandible (jawbone) and other facial bones. Craniates do not include simpler chordates such as lancelets and tunicates. Some craniates are aquatic and have gill slits, unlike the more primitive lancelets which have pharyngeal slits instead. Hagfishes Are the Most Primitive Among craniates, the most primitive is the hagfishes. Hagfishes do not have a bony skull. Instead, their skull is made up of cartilage, a strong but flexible substance that consists of the protein keratin. Hagfishes are the only living animal that has a skull but lack a backbone or vertebral column. First Evolved Around 480 Million Years Ago The first known craniates were marine animals that evolved about 480 million years ago. These early craniates are thought to have diverged from lancelets. As embryos, craniates have a unique tissue called the neural crest. The neural crest develops into a variety of structures in the adult animal such as nerve cells, ganglia, some endocrine glands, skeletal tissue, and connective tissue of the skull. Craniates, like all chordates, develop a notochord that is present in hagfishes and lampreys but which disappears in most vertebrates where it is replaced by the vertebral column. All Have an Internal Skeleton All craniates have an internal skeleton, also called an endoskeleton. The endoskeleton is made up of either cartilage or calcified bone. All craniates have a circulatory system that consists of arteries, capillaries, and veins. They also have a chambered heart (in vertebrates the circulatory system is closed) and a pancreas and paired kidneys. In craniates, the digestive tract consists of a mouth, pharynx, esophagus, intestine, rectum, and anus.   The Craniate Skull In the craniate skull, the olfactory organ is located anterior to the other structures, followed by paired eyes, paired ears. Also within the skull is the brain which is made up of five parts, the romencephalon, metencephalon, mesencephalon, diencephalon, and telencepahlon. Also present in the craniate skull are a collection of nerves such as the olfactory, optic, trigeninal, facial, accoustic, glossopharygeal, and vagus cranial nerve.   Most craniates have distinct male and female sexes, although some species are hemaphroditic. Most fish and amphibians undergo external fertilization and lay eggs when reproducing while other craniates (such as mammals) bear live young. Classification Craniates are classified within the following taxonomic hierarchy: Animals Chordates Craniates Craniates are divided into the following taxonomic groups: Hagfishes (Myxini) - There are six species of hagfishes alive today. Members of this group have been the subject of much debate about how they should be placed within the classification of chordates. Currently, hagfishes are considered to be most closely related to lampreys.Lampreys (Hyperoartia) - There are about 40 species of lampreys alive today. Members of this group include northern lampreys, southern topeyed lampreys, and pouched lampreys. Lampreys have a long, slender body and a skeleton made of cartilage.Jawed vertebrates (Gnathostomata) - There are about 53,000 species of jawed vertebrates alive today. Jawed vertebrates include bony fishes, cartilaginous fishes, and tetrapods.