3.Describe each of the features of Phylum Chordata.
4.Lists the various Subphyla of the Phylum Chordata.
5.Describe the basic features of each of the subphyla
1.2. Introduction
One of the interesting evolutionary feature in the evolution of animals is the evolution of a notochord. The notochord is a stiff elongated structure that is dorsally situated. Group of animals that possess notochord at certain stage of their life cycle are called chordates. Examples of chordates are fishes, amphibians, reptiles, birds, mammals, tunicates (Urochordata), and lancelets (Cephalochordata).
1.3. Characteristics of the Chordates.
Generally, all chordates exhibit or possess five basic features at a certain stage of their life. These features are:
1.Notochord.
2.Dorsal hollow nerve cord.
3.Pharyngeal gill slits.
4.Blocks of muscle.
5.Postanal tail.
1.3.1. The notochord
The notochord is a stiff, flexible rod that is situated dorsally and runs from the anterior part of the body to the posterior. Its main role is givig the support of the body and shape to chordates. The notochord also provides a place for muscles attachment. In higher chordates, vertebrates the notochord is modified into the skeletal system. All chordates possess notochord at a certain stage of their life cycle. The notochord might be present at the adult , larval or embryonic stage.
Figure 1. Characteristics of the Chordates
1.3.2. Dorsal Hollow Nerve Cord
Dorsal hollow nerve cord is a fluid-filled tube of nervous tissue that is situated on top of the notochord and runs from the antherior to posterior part of the body. It is present in chordates throughout embryonic and adult life. Its main function is the control and coordination of activities of the chordates. The dorsal hollow nerve chord is transformed into the brain and spinal cord in the higher vertebrates such as fishes, amphibians, man etc.
Figure 3. Dorsal hollow nerve cord
1.3.3. Pharyngeal gill slits and Arches
Pharyngeal gill slits/clefts are paired openings found on each side of the head region of some chordates. They lead into the pharynx. They are used as filter-feeding organs by invertebrate chordates like lancelets and tunicates. Juvenile fishes use them for breathing In adult fishes, the pharyngeal gill slits develop into true gills. While in reptiles, birds, and mammals the gill slits became vestigial, occurring only in the embryo. However, they are transformed into the ear ossicles(organ of hearing) in the adults ( man).
Figure 3 pharyneal clefts
1.3.4. Post-anal tail
Post anal tail is the extension of the posterior part of the body. All chordates, including man possess tail at certain stage of their life cycle. The tail may be present at the adult, larval or the embryonic stage.
Figure 4: Post anal tail
1.3.5. Myotomes
Myotomes are block of muscles surrounding the notochord and nerve cord. These are transformed into the muscular system in higher chordates. Its main function is support, shape, movement etc
Figure 5: Myotomes
1.4. Classification of the Phylum Chordata.
Member of the Phylum Chordata are classified into three subphyla. These are:
1.4.1. Subphylum Urochordata
1.4.2. Subphylum Cephalochordata
1.4.3. Subphylum Vertebrata
1.5. Module summary
You have learnt the followings from this module:
- Chordates are group of animals that possess an elongated, supporting structure called the notochord at certain stage of their life.
- All chordate most possess notochord, dorsal hollow nerve cord, pharyngeal clefts, post anal tail and myotomes at certain stages of the life.
- Chordates are classified into three sub-phyla; Urochordata, Cephalochordates, and vertebrata