This great phylum (Phylum chordate) derives its name from one of the few common characteristics of the group – the notochord. This structure is possessed by all members of the phylum either in the larval or embryonic stages or throughout life. The notochord is a rod-like semi rigid body of vacuolated cells which are filled with proteinaceous material which extends in most cases the length of the body between enteric canal and the dorsal hollow central nervous system. Its primary purpose is to support and to stiffen the body that is to act as skeletal axis.
It seems that the endoskeleton is the chief basic factor in the development and specialization of higher animals.
The animals most familiar to us belong to the chordates including man himself.
The chordates show great variety and inhabit all kinds of habitat. All chordates possess three basic characters which are as follows:
- As already mentioned all possess the notochord.
- All chordates have central nervous system that is dorsal in position and is hollow.
- All chordates develop paired gill openings in embryonic stage. In some these are non-functional. While in others they are functional for some period in their life history e.g. frogs etc. in still other these are functional throughout life e.g. amphioxus, and fishes etc.
phylum Chordates have been divided into lower chordates. e.g. Amphioxus etc. and higher chordates which are the vertebrates in which the notochord is replaced by the vertebral column and a bony brain case cranium is also formed due to which they are also called craniates. Phylum Chordata has been sub-divided as follows:
Protchordata (Acrania) (Lower Chordates)
Sub-phylum: Urochordata: Notochord and nerve cord only in the free-swimming larvae. Adults are sessile and enclosed in a covering called tunic. Therefore they are also called tunicates e.g. Molgula.
Sub-phylum: Cephalochordata: Notochord and nerve cord extend along the entire length of the body and persist throughout life e.g. Amphioxus.
Classifccation of Phylum Chordates