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Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. Placoderms were among the first jawed fish; their jaws likely evolved from the first of their gill arches. A 380-million-year-old fossil of one species represents the oldest known example of live birth. [18]
The mouth is located just behind the head, after a rostrum, on the underside of the body. It has a pair of cuticular jaws, supplied by strong muscles, and often bearing minute teeth. A "basal plate" on the lower surface that bears a comb-like structure is also present.
Acanthodii or acanthodians is an extinct class of gnathostomes (jawed fishes).They are currently considered to represent a paraphyletic grade of various fish lineages basal to extant Chondrichthyes, which includes living sharks, rays, and chimaeras.
Regions of a Chondrichthyes brain colored and labeled on dissected skate. The rostral end of the skate is to the right. In chondrichthyans, the nervous system is composed of a small brain, 8–10 pairs of cranial nerves, and a spinal cord with spinal nerves. [11] They have several sensory organs which provide information to be processed.
Brain at the U.S. National Library of Medicine Medical Subject Headings (MeSH) (view tree for regions of the brain) BrainMaps.org; BrainInfo (University of Washington) "Brain Anatomy and How the Brain Works". Johns Hopkins Medicine. 14 July 2021. "Brain Map". Queensland Health. 12 July 2022.
The head rests on the top part of the vertebral column, with the skull joining at C1 (the first cervical vertebra known as the atlas). The skeletal section of the head and neck forms the top part of the axial skeleton and is made up of the skull, hyoid bone, auditory ossicles, and cervical spine. The skull can be further subdivided into:
The plates most likely had a regulation function for the volume of the lung. [39] Due to the size of the fatty organ, researchers assume that it is responsible for the kidney's unusual relocation. The two kidneys, which are fused into one, [ 40 ] are located ventrally within the abdominal cavity, posterior to the cloaca.
The subgranular zone (in rat brain). (A) Regions of the dentate gyrus: the hilus, subgranular zone (sgz), granule cell layer (GCL), and molecular layer (ML). Cells were stained for doublecortin (DCX). (B) Closeup of subgranular zone, located between the hilus and GCL, [33] a site of adult neurogenesis. Phenotypes of proliferating cells in the ...