Quiz 23: Chordates

Biology

The characteristics that are shared by the three Deuterostome phyla that indicate a monophyletic group are listed below- • They all show radial cleavage during development. • Their anus is derived from the first embryonic opening that is blastopore. • Their mouth is derived from an opening of secondary origin. • Coelom formation is by Schizocoely.

The cladistics classification of the vertebrates produces important regroupings of the traditional vertebrate taxa. Examples: • Agnatha includes vertebrates which lack jaws, have features of anamniotes, that is, vertebrates which do not develop within a fluid-filled sac • Tetrapoda include vertebrates that have four limbs. Now there are some tetrapods that are amniotes and some are anamniotes • Amniotes include tetrapods with embryos that have extraembryonic membranes • Bony fishes and tetrapods together compose teleostomi Hence, this regrouping is possible by looking at the cladistics classification of the vertebrates. Agnatha and Reptilia are inconsistent with cladistic principles because they fail to satisfy the cladistics requirement. They are paraphyletic due to which they are not accepted in cladistics classification. Paraphyletic refers to those which originate from different phylogenetic groups. Hence, Agnatha and Reptilia are no longer recognized in cladistics usage.

The five hallmarks shared by all chordates are as given below- • Notochord- It is a rod like flexible structure extending through the length of the body. It is an axis for muscle attachment as it can bend without shortening. This permits undulatory movements of the body. It may not be present throughout the life of chordates but is visible at least at some stage of development. • Dorsal tubular nerve cord- All chordates have a tubular or hollow nerve cord present on the dorsal side of gut. The anterior end of nerve cord is enlarged to form brain in craniates. • Pharyngeal pouches and slits- Pharyngeal slits are openings that lead from the pharyngeal cavity to the outside. They are formed when the two pharyngeal pouches meet. These pharyngeal pouches are formed by the evagination of the endodermal lining of the pharynx. In tetrapods, the pharyngeal pouches give rise to several structures including Eustachian tube, the middle ear cavity, tonsils and parathyroid glands. The perforated pharynx evolved as a filter feeding apparatus and is used as such in protochordates. Aquatic vertebrates possess internal gills which are an adaptation of the pharynx. • Postanal tail- A postanal tail is present at least at some stage of life cycle in all chordates. It acts as a balancing organ and aid in locomotion. • Endostyle or thyroid gland- Earlier, the Endostyle was not considered as a chordate character but now it is known that it or its derivative the thyroid gland is found in all chordates but in no other animals. The Endostyle is present in the pharyngeal floor and secretes mucous which traps food particles. Some cells in the Endostyle secrete iodinated proteins. These iodinated protein secreting cells are homologous to the iodinated hormone secreting cells of the thyroid gland. In primitive chordates, Endostyle and perforated pharynx work together to create an efficient filter-feeding apparatus.

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