Categorization and General Features of Chordates:

The characteristic features of chordates are as follows:

i) The notochord is present at some stage in the life history of chordate

ii) They are triploblastic coelomates

iii) Bilaterally symmetrical

iv) There are visceral (pharyngeal) clefts present.

v) The dorsal hollow nerve chord is also present

vi) There are segmental muscle blocks (myotomes) on either side of the body

vii) There is a post anal tail

viii) Limbs are created from more than 1 body segment.

Protochordata:

Majority of chordate are vertebrates. Other subphylum of chordates which are not vertebrates are protochordates that have some invertebrate's characteristics as well as vertebrate features. This recommends that they are evolutionary connection in invertebrates and vertebrates. Amphioxus and trinicates are supposed to be only animals connected with vertebrates who lack vertebral column. Amphioxus has the notochord, segmented musculature, and dorsal nerve cord. It doesn't have brain and sense organs. They live submerged in sand or mud. They don't have true jaws. They have suckerlike mouth. They absorb water from body surface and are capable to move about. Group of chordates which have vertebral column belong to subphylum vertebratel are as follows:

General features of Vertebrates:

i) The notochord is replaced in the adult by a vertebral column.

ii) There is a well developed central nervous system including brain

iii) The skeleton is internal

iv) The pharyngeal clefts are few in number

v) There are two pairs of fins or limbs.

Factors of categorization of Vertebrates:

Vertebrates are group into 6 classes:

• Chondricnthyes
• Osteichthyes
• Amphibia
• Reptilia
• Aves
• Mammalia

Evolutionary Developments in Vertebrates:

In terms of size and power, vertebrates have develop into dominant or earth. They have adapted to land, sea, and air and developed in very different ways.

Apart from insects, vertebrates are governing creatures on earth. They have primitive jawless fishes, true fishes, amphibians, reptiles, birds and mammals. In phylum chordate also have 2 groups of animals which are not vertebrates. These are 2 marine group's tunicates and lancelets. There is kind of progression in growth from primitive vertebrate structure-fish, through amphibians and reptiles to more advanced ones birds and mammals. Although any fish is compared to the amphibian, for instance, as amphibians signify stage more adapted to life on land than fish, fish is somewhat specialized anyway. Likewise the reptile is more adapted to life on land than the amphibian and so on till we meet the mammals.

The Basic Chordate Body Plan:

This body plan has notochord, above which is tubular nerve cord. There are pared gills slits that open from throat to exterior of body. There is heart situated on more ventral part of body. It is only in vertebrates which the series of bony or cartilaginous vertebrae grow round notochord. These strengthen the notochord but retain the flexibility. Also related with these are the sequences of blocks of muscle tissue on either side of body utilized in swimming. This reflects segmentation that is also apparent in arrangement of spinal nerves and arteries, vertebrae, and even few excretory organs. The gills were seen as mainly apertures by which excess water allows in through mouth of feeding or swimming animal are passed out. Participation of gills with respiration appears the secondary development. In aquatic vertebrates locomotion is influenced by muscle contraction began in muscle segments starting from head, and transmitted as the wave passing through rest of body. In ancient times jawless fish simply single longitudinal dorsal and ventral fin used as stabilizer in forward movements. In true fishes there are 2 sets of pairs fins, every pair joined to the pharyngeal slit pharynx internal supporting skeletal girdle. These used as extra stabilizers and control. Entire variable jaws are also there in true fishes.

General Characteristics:

Class - Chondricthyes (cartilaginous fishes):

These are cartilaginous fishes. They possess skins with tooth-like (placoid) scales. There skeletons are composed of cartilage. They possess paired pelvic and pectoral fins. They possess no air bladder. Asymmetric tail fin assists to avoid animal from sinking. 5 pairs of visceral clefts live as separate gill openings. There are no outer ears. Eggs are made and fertilization is internal. Examples are shark, dog-fish, rays and skates.

Class - Osteichthyes:

These are bony fishes. Their skins are enclosed with thin, surrounding by scales. These scales are explained as cycloid. Their skeleton is composed of bones, unlike cartilaginous fishes. Paired pectoral and pelvic fin are present maintained by bony rays. This creates it simpler to maneuver. Dissimilar in cartilaginous fish, fish tail of bony fishes are symmetrical. 4 visceral clefts are here as 4 separate gill openings on either side of head. They are though covered by the bony flap known as operculum. There are no outer ears.

Adaptations of Fish:

Fishes are usually streamlined and have rather rounded head end. Streamline body is developed for moving through dense water medium with minimum friction and turbulence. Head has sensory and feeding structures. Rest of the body has locomotive muscles which propel fish through water. Lateral and vertical fins act as stabilizers. As they are predators (carnivores) they contain jaws. Body of fish is relatively large, compact and energetic. These 3 features demand the effective maintenance organization. Upper body is usually segmented while lower part is not segmented and surrounds the large body cavity that is the true coelom. Segmented muscle organization provides the locomotive benefit and room for development and complicated digestive system. Due to this arrangement, fishes have room to increase to fulfill demands of the compact, active and bulky body.

Evolution of the Vertebrate Jaws:

Placoderms are initial true fishes with true jaws. The jaws included the expanded pair of hinged gill supports with 2 skeletal bars on every side of throat, hinged together on side at connection of 2 bars. It is also hinged underneath with lower bar of opposite side. Every bard is tilted frontward and extended. This slant causes formation of the angle for jaws. Lower connection now becomes front end where lower jaws meet. Upper bar becomes upper jaw on every side and are joined to skull by muscles which operate them. This growth into jaws of the gill supporting skeletal system is observed in growth of all embryos of vertebrates - from fish to man.

The Maintenance Systems:

1) Food Intake:

Food and water is taken in through large mouth of fish and is swallowed. On reaching throat water get away through series of gill slits. Walls of slits are much folded to form huge respiratory surfaces. Slits in bony fishes are fewer (4 in number) while in cartilaginous fishes they could be up to 7. Fish have gill rakers that form stiff cross grid for stopping food particles from evading with water. As it moves through water with mouth open, it mechanically drive water through gill slits by muscular action of throat wall of every side, that alternately extends and contracts throat. In cartilaginous fishes every gill slit opens separately to exterior, while in bony fishes operculum covers gill openings on every side, forming the gill chamber. Jaws serve to open and close mouth; to seize and hold food jaws look like gill arch except that jaws are bigger and have attachment to base of skull. Upper and lower jaws are known as first (mandibular) and second (hyoid) arch respectively. Digestive System comprises of moult with its jaws, teeth, tongue and oesophagues, the large stomach developed to accommodate the large rapidly ingested meal in which enzymes secreted by infolding walls break down protein actually and chemically. At last in large intestine, food residues are stored until they are eradicated.

2) Circulatory System:

Vertebrate blood is rich in concentration of hemoglobin. Oxygen and nutrients are absorbed from small intestine are distributed round body by the well-developed circulatory system. Fish's circulatory system is composed of blood, heart and blood vessels. Heart is situated at mid ventral region of body. In crustaceans heart is in mid dorsal region. Fish heart is 2-chambered, fore and in back pump that draws blood from behind and pushes it forward. Posterior chamber is known as atrium (auncle) and anterior chamber (the ventricle). Venus sinosus obtains venons blood and pumps it forward to gills by ventral aorta. Major arteries pass from ventral aorta to gills on every side and are numbered according to gill arch they provide.

3) Excretion:

The kidneys are excretory organs. They situate in upper part of body cavity attached one on either side of dorsal aorta. Arterial blood from dorsal aorta enters it under quite high pressure. The percentage of blood nutrients are filtered at glomerulus. Blood leaves kidney by renal vein and from there to heart. Liquid filtered gather in long 5 kidney tubules that finally unite to form excretory or nephric duct of every kidney. Reabsorbtion takes place as substances pass through tubules and stored in blood system. Excretory ducts run along upper part of body cavity and come to open rear arms at cloaca. Body cell and tissues are of the greater concentration than fresh water in environment. This means there will be constant water intake which kidney should eliminate from body. As filter pumps of kidney tubules are not selective, they too reabsorb definite substances as filtrates pass down tubes.

4) Locomotion:

For fish to move quickly through very resistant means like water, it should have necessary features of leverage, shape, stabilizing fins and automatic regulators. The adaptations of fist to locomotion in water:

a) The streamlined body-tapering at both ends, move to tail then at head. This provides least resistance to water and so lesser energy is extended in trying to pass through water.

b) It pushes down against resisting water with its tail fin. This develops the leverage that pushes animal forward.

c) The tail moves from side to side, therefore causing body to move under pressure of water that tail is always in the slant up position relative to body axis. As blade pushes water away in any direction, reward is the forward movement.

d) Efficiency of forward movement relies on size, relative size of tail fins, slant of blade and speed at which it is moved.

e) Much of forward thrust of fist comes from side to side movements caused by muscles rather than that of tail fin.

f) Muscles run parallel to side of body, from head to tail. When those on one side contrast, others on opposite side relax, causing body to bend towards side of contracting muscles.

The fish also contain vertical fins that provide keels preventing from loosing balance when it creates rapid turns. 2 pairs of horizontal fins extending sideways from body, pectoral and pelvic fins (articulate with internal skeletal support) function as control for up and down pitching movement of head and body, maintaining whole animal on level course.

5) The Senses:

The eventual source of external stimuli is senses which comprise stretch receptors, eye, organs of balance (labyrinth or vestibular apparatus) and lateral like. Their functions are as follows:

a) Eyes relate animal's direction of movement to a few external objects.

b) Stretch receptors or muscle spindles are situated in muscle tissue all through body. They reply to contractile changes in muscles. Brain and spinal cord mediate both the incoming and outgoing impulses to effect the dynamic self regulation.

c) Labyrinth is situated on either side of head immediately behind eyes. They keep equilibrium during movement. They have three semi-circular canals (all filled with fluid) prearranged at right angles to one another in order that where ever fist turns it can have sense of balance. This is made through nerve impulse joined to brain.

d) Lateral line is the extra device for fish to perceive water pressure at body surface. All fists have this line as the organized system of sensory canals in skin which open at intervals to surface. This system extends whole lengths of body.

6) The Head:

The head has numerous functions. Due to its shape, rigidity and fact which it bears sense organs related with movement by water, and orientation. It also contains parts that are liable for food and water intake.

The skull that houses brain is cartilaginous, tough and flexible. It is reinforced from outside by scales, bones or mineral deposits. Head has many segments combined form dense and firm structure better able to work as the unit.

7) The Brain:

It is front part of spinal cord. It is extended to include cranial nerves, sense organs and activity of jaws. Brain parts are prearranged in same sequence as parts of body they manage.

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