Zoology · Ch 4 — Animal Diversity-II: Phylum Chordata
General Characters of Chordata
General Characters of Chordata
General Characters of Chordata
Phylum Chordata is defined by four hallmark structural features, present at some stage of the life cycle in every member of the phylum:
- A notochord — a flexible, rod-like, longitudinal supporting structure running along the back of the animal, beneath the nerve cord and above the gut. In many chordates the notochord is present only in the embryo and is later replaced by a vertebral column (backbone), while in a few groups it persists into adulthood.
- A dorsal, tubular (hollow) nerve cord — the main nerve cord runs along the back of the animal, above the notochord, and is hollow (tubular) in cross-section, in sharp contrast to the solid, ventrally placed nerve cord found in essentially every non-chordate phylum with a nerve cord (such as Annelida and Arthropoda).
- Pharyngeal gill slits — a series of paired openings connecting the pharynx (throat region) to the exterior, present at some stage of development in every chordate, even those (like most reptiles, birds and mammals) in which they appear only transiently in the embryo and never function as breathing structures in the adult.
- A post-anal tail — a tail extending behind (posterior to) the anus, present in every chordate at some point in its life, even in groups such as humans where it is reduced to a small vestige (the coccyx) in the adult.
These four characters, taken together, are what unites the very different-looking members of Chordata — from a sea squirt, to a fish, to a bird, to a human — into a single phylum, since no single non-chordate phylum shares this particular combination of features. Chordata also shares certain deeper developmental features with phylum Echinodermata: both are deuterostomes, in which the blastopore develops into the anus, cleavage is radial and indeterminate, and the coelom forms enterocoelously (as outpocketings of the embryonic gut) — placing Chordata and Echinodermata together, along with Hemichordata, as the three deuterostome phyla, despite their very different adult body plans.
What this figure shows. A generalised diagram of the chordate body plan showing the relative positions of the notochord, the dorsal hollow nerve cord above it, the pharyngeal gill slits, and the post-anal tail.
Figure 1: Chordate Body Plan.