The Truth Behind....
Cephalocordata and Urochordata
What Is A Chordate?
Urochordatata and Cephalochordata are subphyla of the phylum Chordata. They belong to Kingdom Animalia in the Domain Eukarya. Phylum Chordata is the most diverse phylum in the animal kingdom. In addition to Urochordata and Cephalochordata, Phylum Chordata contains the subphylum Vertebrae, which includes humans and their closest relatives. Chordates are active animals with bilaterally symmetric bodies that are longitudinally differentiated into head, trunk and tail (Lundberg 1995).There are four distinguishing anatomical features of Phylum Chordata, some of which are only apparent during embryonic development (Campbell et al. 1999). These features include:
1) A notochord (a flexible rod between the nerve cord and digestive tract)
2) A dorsal, hollow nerve cord
3) Pharyngeal slits
4) Muscular, postanal tail
Read on to learn more about the distinguishing anatomical features of Phylum Chordata.
The Notochord
A notochord is defined as a longitudal, flexible rod located between the digestive tube and the nerve cord. The notochord serves as the skeletal structure and support of all chordates, and it is from the notochord that chordates derive their name. Made of "large, fluid-filled cells encased in stiff, fibrous tissue," the notochord is prevalent in nonvertebrate chordates, while most vertebrates develop a "more complex, jointed" skeleton (Campbell et al. 1999). When the muscles of the notochord contract, they cause the chordate to bend (Paisley).
http://www-biol.paisley.ac.uk/courses/Tatner/biomedia/subunits/notoc.htm
(permission pending)
A Dorsal, Hollow Nerve Cord
The dorsal, hollow nerve cord is unique to chordates and develops from an ectoderm plate that "rolls into a tube located dorsal to the notochord." This nerve cord develops into the central nervous system of the brain and spinal cord. In contrast, most other animal phyla have ventrally located, solid nerve cords (Campbell et al. 1999). The dorsal nerve cord lies directly above the notochord (Paisley).
http://www-biol.paisley.ac.uk/courses/Tatner/biomedia/subunits/nerve.htm
(permission pending)
Pharyngeal Slits
Pharyngeal slits are important in the chordate feeding and digestive process. The pharyngeal slits serve as an opening to the pharynx which is located posterior of the chordate mouth, along the digestive tube (Campbell et al. 1999). Up to 200 vertical slits can perforate the wall of the pharynx, and each slit is separated by stiffening rods (Paisley). Rows of cilia, a hair-like organelle, force water to flow into the mouth, through the pharyngeal slits and out of the body through the antipore, a hole located at the end of the body (Paisley).
http://www-biol.paisley.ac.uk/courses/Tatner/biomedia/subunits/phary.htm
(permission pending)
Muscular, Postanal Tail
The chordate tail serves two purposes. The first purpose of the tail is to envelope the muscles and "skeletal elements" of the body. The second is to provide the "propulsive force" in various aquatic species. The postanal tail extends posterior to the anus. In contrast, the digestive tracks of nonchordates extend over the entire length of the body (Campbell et al. 1999).
The Cephalochordate
The organisms of Subphylum Cephalochordata, also known as lancelets or Amphioxus, are "sand-burrowing marine creatures" each a few centimeters in length (Lundberg 1995). There about approximately 20 species in the cephalochordate group, each possessing a "blade-like shaped" body (Lundberg 1995). Adult cephalochordates have a notochord, dorsal, hollow nerve cord, pharyngeal slits and a muscular, postanal tail, as listed above. Although cephalochordates are able to feebly swim, most of their time is spent partially buried in the sand filtering microscopic food particles from the water (Ultranet). The primary motion of cephalochordates is "wriggling backwards into the sand, leaving only its anterior ends exposed" (Campbell et al. 1999). This sinusoidal movement is a result of a "serial musculature" that allows for "side-to-side undulations" that control the body movement of the cephalochordate (Campbell et al. 1999). These muscular segments develop from a block of mesoderm called somites that are located along the notochord of a chordate embryo (Campbell et al. 1999).
The Cephalochordate
(permission pending)- http://www.ultranet.com/~jkimball/BiologyPages/I/Invertebrates.html#Chordates
The Urochordate
The roughly 1600 marine species of Subphylum Urochordata are commonly known as tunicates, sea squirts, and salps (Lundberg 1995). While urochordates have free-swimming, active larval forms, they undergo a rapid metamorphosis to several different adult states (Lundberg 1995). Although most adult urochordates are sessile and exist adhered to rock, boats and docks, other urochordates are planktonic or exist in colonies (Campbell et al. 1999). While the larva of urochordates have the same basic characteristics of a chordate, adult urochrodates "scarcely resemble" chordates because they show no evidence of a notocord, nerve cord or tail (Campbell et al. 1999). Like cephalochordates, urochordates are filter feeders that live on the sea floor (Lundberg 1995).
The Body of a Urochordate
http://www.phage.org/campbl34.htm#pharyngeal_slits
(permission granted)
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