Chordata , phylum of animals having a notochord, or dorsal stiffening rod, as the chief internal skeletal support at some stage of their development. Most chordates are vertebrates (animals with backbones), but the phylum also includes some small marine invertebrate animals.
The three features unique to chordates and found in all of them at least during early development are:
The notochord, composed of gelatinous tissue and bound by a tough membrane
A tubular nerve cord (or spinal cord), located above the notochord
Gill slits leading into the pharynx, or anterior part of the digestive tract (the throat, in higher vertebrates). In addition, all have blood contained in vessels, and the tunicates and vertebrates have a ventrally located heart. All have a postanal tail, that is, an extension beyond the anus of the notochord or backbone and of the body-wall musculature, containing no internal organs.
In vertebrates, a backbone of bone or cartilage segments called vertebrae develops around the notochord; its upward projections partially surround the nerve cord. In many fishes and in early fossil amphibians and reptiles the notochord persists in the adult and is enclosed by the vertebrae; in higher vertebrates, however, it disappears during embryonic development.
There are two invertebrate subphyla: the Urochordata, or tunicates, and the Cephalochordata, or lancelets. A third invertebrate group, comprising the acorn worms and their relatives, shows affinities with chordates and has sometimes been considered a chordate subphylum, but is now often classified in a phylum of its own, the Hemichordata.
As far as the Indian snakes are concerned, they can be summoned under these families...
Boidae: Sand boas and pythons
Typhlopidae: worm snakes
Xenopeltidae: sun beam snake
Achrochordidae: File snake
Colubridae: common 'non venomous' / rear fanged snakes
Elapidae: cobras, kraits, coral snakes
Hydrophiidae(sometimes treated as sub family hydrophinae of Elapidae): Sea snakes
Viperidae: True vipers and pit vipers
The families Elapidae, Viperidae, Colubridae and Attractospididae form a clade known as advanced snakes. All these families, including the so-called non-venomous snakes (Colubridae) have venom encoding genes. However, most of the members of Colubridae lack a sophisticated venom delivery apparatus and/or have venom that is not lethal against humans. Hence, they are commonly referred to as non-venomous snakes. One should remember that most of these colubrids are not at all dangerous to humans (with the exception of rear-fanged snakes like Boomslang). In India, there are no colubrids that can be lethal to humans.
Snakes that could be dangerous to man usually fall into two groups . . .
The proteroglypha have, in front of the upper jaw and preceding the ordinary teeth, permanently erect fangs. These fangs are called fixed fangs.
The solenoglypha have erectile fangs; that is, fangs they can raise to an erect position. These fangs are called folded fangs.
The fixed-fang snakes (proteroglypha) usually have neurotoxic venoms. These venoms affect the nervous system, making the victim unable to breathe.
The folded-fang snakes (solenoglypha) usually have hemotoxic venoms. These venoms affect the circulatory system, destroying blood cells, damaging skin tissues, and causing internal hemorrhaging.
Remember, however, that most poisonous snakes have both neurotoxic and hemotoxic venom. Usually one type of venom in the snake is dominant and the other is weak.
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