Explain the classification of phylum chordata?

The phylum chordata is divided into different sub phyla. These are known as the urochordata, cephalochordate and vertebrata. They all have different features and they are described below.

The urochordata is also known as the tail notochord. They are also known as tunicata, as their body is enclosed in a tunic which is composed of the cellulose. In this case the notochord is present in the embryo stage. It disappears in the adult stage. It occurs in the tail portion. The larva also contains a dorsal nerve cord. However, in the adult it is replaced by a dorsal ganglion. There are numerous apertures in the pharynx known as stigmata. They are formed by the division of gill slits. The stigmata have an opening in the ectoderm referred as atrium. The larva undergoes certain changes known as metamorphosis. It is of retrogressive type which means the larva changes from better developed to less develop. The cephalochordate is also known as the head notochord. In these organisms the notochord extends till the anterior part of body. In this case the notochord is present throughout the life. It is also present in the adult stage. There are numerous apertures in the pharynx known as stigmata which are better developed than the urochordata. The stigmata have an opening in the ectoderm referred as atrium. The tail is also present throughout their life. Amphioxus is also known as lancelet as their both the ends are pointed.
The subphyla mentioned above are without the cranium. These are known as acrania. They are also referred as the primitive chordates.

The vertebrata are also known as craniata. They have cranium. In this case the notochord is present in the embryo stage. It disappears in the adult stage and is replaced by the vertebral column. They have an organ system level of organization. The digestive system is complete and has mouth and anus for ingestion and egestion. They have a couple of appendages. They contain muscles of all the origins such as striped, unstriped and cardiac. They have well developed coelom. The formation of head also occurs. The epidermis is multilayered. It gives rise to feather, scales and hairs. They are referred as an exoskeleton. The endoskeleton is formed by the cartilage or bone or by both. The process of excretion and respiration occurs by a pair of kidneys and gills, lungs, skin and bucco pharyngeal cavity respectively. They have multiple cranial nerves. They have no form of asexual reproduction. They reproduce sexually. They may or may not show sexual dimorphism. They have the gonads whose number varies from one to two. The development can be indirect with the formation of larva. It has internal as well as external mode of fertilization. They have the multiple sense organs which include eye, ear, tongue and skin. They have the endocrine glands. They have a hepatic portal system. Some of them have a renal portal system and pituitary portal system. They have a circulatory system which is of closed type. It has blood vascular system and a lymphatic system. The lymph is colorless and is similar to blood. The blood has red and white blood cells. The red blood cells contain haemoglobin. The white blood cells play a defensive role. They have different types of nervous systems. It includes the central nervous system, peripheral nervous system and autonomic nervous system. The central nervous system is composed of brain and spinal cord. The peripheral nervous system consists of cranial and spinal nerves and autonomic nervous system is made up of the sympathetic and parasympathetic system.
The vertebrata are divided into agnatha and gnathostomata.