Subphylum

Craniata

SubphylumId   4
Subphylum   Craniata
Description   Animals with skulls; Fishes, amphibians reptiles, birds & mammals
Year   1758
AuthorWI   Linnaeus [C.]
AuthorNI   27
Notes   Craniata replaces Vertebrata at the Subphylum level. Vertebrata falls within Craniata as an unranked taxon. Fishwisepro has placed it into Infraphylum to give it a rank. For Craniata: Notochord never extends in front of the brain; cranium present; vertebrae usually present; cartilage or bone or both are present; heart chambered; red blood corpuscles usually present; brain well developed; 10 to 12 cranial nerves; dorsal and ventral nerve roots usually united; nephridia absent; epidermis with several cell layers; endostyle only in larval lamphreys (ammocoetes) and transformed into thyroid tissue in all others; sensory capsules present; neural crest formation present. The Craniata, or craniates, include all animals having a skull (or cranium, hence their name), be it cartilaginous or bony. A skull is a box of hard tissues which encloses the brain, olfactory organs, eyes, and internal ear. Craniates comprise all fishes - including such jawless fishes as hagfishes and lampreys - amphibians, reptiles, birds and mammals, including Man. The earliest known undisputed craniates are jawless fishes which lived 480 millions years ago. Their evolutionary history took place first in the sea, then in fresh waters. Some craniates, the tetrapods or four-legged vertebrates, became terrestrial and arose about 370 millions years ago from fish ancestors. Now, the majority of the craniate species are represented by one group of fish, the actinopterygians, and the tetrapods. Other craniate groups (jawless fishes, sharks and chimeras, the coelacanth, lungfishes) are considerably depauperate, by comparison with their past diversity. The Craniata fall into two major clades, the Hyperotreti, or hagfishes, and the Vertebrata. Since the early nineteenth Century, and until recently, the hagfishes were regarded as the sister-group of lampreys (Hyperoartia). The two groups were gathered into the clade Cyclostomi (see below: Discussion of phylogenetic relationships). However, lampreys share with jawed vertebrates, or Gnathostomata, a large number of morphological and physiological characteristics that occur neither in hagfishes, nor in non-craniate chordates (cephalochordates and tunicates). There is, therefore, a rather broad consensus over the theory that hagfishes are the sister-group of the Vertebrata (lampreys and gnathostomes). The Craniata are characterized by a skull (initially cartilaginous and fibrous), which includes three types of sensory organs derived in ontogeny from ectodermal placodes; that is, thickened patches of the embryonic skin that sink inward toward the brain where they develop into sensory chambers. Anteriormost of these is the olfactory organ, which is initially unpaired, and becomes paired in the Vertebrata. Behind it are the paired eyes, the photoreceptors that develop as lateral outgrowths of the brain. The skin and connective tissues adjacent to the neural (photoreceptive) part of the eye add secondary structures in the Vertebrata (lens, intrinsic muscles, and eye lids). Posteriormost of these sensory organs in the head are the paired acoustic organs or inner ears. The inner ears are mechanoreceptors concerned with hearing, balance, and perception of position of movement. The sensory cells of the inner ear are enclosed in a cavity filled with a liquid, the endolymph, and which develops from one to three semicircular canals. The acoustic organs also comprise a special component, the lateral sensory system, which is lost in most terrestrial craniates (Amniota). It consists of lateralis nerve fibres derived from the acoustic nerve and superficial mechanoreceptors, the neuromasts, which are housed in grooves or canals on the surface of the head. These extend onto the body in the Vertebrata. True neuromasts, however, seem to be unique to the Vertebrata, and have never been observed in hagfishes. - All animals having a skull (or cranium), whether it is cartilaginous or bony.
Phylum   Chordata
Uacd   a
Last updated   1/1/2018 12:00:00 AM
User   Dennis Polack
DeleteInd   False
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