Category Archive Animal diversity (Nonchordata)

Animal Diversity (Non-Chordata)



Non-chordates are animals without a notochord. They are the most abundant and diversified of all animals living or extinct. That makes their study the most fascinating one. 

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Echinoderms can voluntarily and rapidly change the stiffness of their connective tissue, which is called mutable connective tissue. Their bodies can become stone hard or in holothurians it can become so soft that it may flow between fingers.

If attacked by a predator, brittle stars can break their arms at will and grow them again later. This is called Autotomy.

Brittle stars do not have intestine, anus, dermal branchiae, pedicellariae and ambulacral grooves.

Echinoderms have no brain or ganglia, and nerves are made of diffused neurons.

Sea urchins can harden or soften their spines at will.

Starfishes are slowest of predators that take 4-8 hours to kill and consume a mollusc prey.

Aristotle’s Lantern is actually a set of masticatory jaws of sea urchins for feeding on algae from rocks.

Pelagothuria is a pelagic holothurian echinoderm that can swim like a jelly fish with the help of webbed papillae.


Mollusca means soft bodied, although it includes animals having hard shell.

Neopilina galatheae, collected from 3500 m depth is a connecting link between Annelida and Mollusca.

Brachystomia is a tiny shelled snail that sucks blood of clams.

Conus is a predatory snail that lures a fish by a bait (its worm-like modified proboscis) and stings fish’s tongue when it tries to eat the bait. The paralysed fish is then swallowed by the snail.

Foot of sea butterflies (Pteropods) is modified into wing-like parapodia which are used for active swimming.

Giant clam (Tridacna maxima), found in the coral reefs of Indo-pacific region, is 1.5 m long and weighs 225 kg.

Entovalva is the only parasitic bivalve mollusc that lives inside the gut of sea cucumbers. Entochoncha is a worm-like parasitic gastropod that also lives in the body cavity of sea cucumbers.

Glochidium larva of bivalves is parasitic on the gills of fishes.

Scallops (Pecten) can swim by using its valves as wings.

Teredo and Bankia are wood-boring bivalves that damage boats.

Cephalopods have 3rd or 4th arm hectocotylized (or modified spoon-shaped) which is used to introduce sperms into the mantle cavity of female.

Squids, cuttle fishes and Octopus possess strong beak-like jaws often laced with poisonous saliva that makes them fierce predators of the sea.

Giant squids (Architeuthis; Mesonychoteuthes) of the Pacific are the largest invertebrates which are over 50 ft long and weigh more than 500 kg. They live at a depth of 2-3 kilometers in pacific ocean.

Giant Octopus (Octopus dofleini), which inhabits pacific ocean, has a arm-span of ten meters.

Cephalopods swim by a faster jet propulsion method.

Cephalopods’ brain is more advanced than any other invertebrates that makes octopus the most intelligent invertebrate that can carry out complex tasks.

Cephalopod eyes are strikingly similar to vertebrate eyes but have evolved independently, which a strange coincidence.

Argonauta is a cousin of octopus but secretes an external shell around the body.

Largest eye in the animal kingdom, having a diameter of 12 inches, belongs to the giant squid (Mesonychoteuthis). Its lens is the size of a tennis ball.

Snails are born bilaterally symmetrical but within 15 minutes they become spirally coiled by torsion.

Pearl oysters secrete pearl around any object that is trapped in their mantle cavity and causes irritation.


Arthropods are the most successful of all animals on earth. They also make more that 80% of animal species.

Japaneses spider crab (Macrocheira kaempferi) is the largest living arthropod. Its legs are 5-6 feet long.

Trilobites were the earliest arthropods which have all become extinct. Over 4000 species of fossil trilobites are known today.

More than 1,000 species of sea spiders (Pycnogonida) live up to 6,000 m depth and feed on cnidarians and worms.

Giant water scorpions (Eurypterida) grew up to 3 meter length and were the top predators of the Paleozoic era.

Giant centipedes (Scolopendra) can attain a length of 30 cm and their bite can be fatal to humans.

Tracheal system of insects is a unique respiratory system that conveys oxygen directly to the muscles where it is needed. Therefore, there is no need of respiratory pigment in the blood of insects.

Compound eyes are unique innovation of arthropods. The eye is made of thousands of ommatidia, each of which functions independently. Compound eye is superior because it gives 360 degrees of visual field, a much sharper vision and more depth of field at higher magnification, and detects movement of predators no matter how fast it is.

Insects are the most abundant and most successful of all creatures on earth; they make three-fourth of all animal species and also the most species that survived successive mass extinctions.

Male and female scorpions have a honeymoon dance called “Promenade a deux”, after which female kills and devours the male. Similar phenomenon is seen among spiders, in which male is much smaller than the female.

Termites are the earliest social animals, which developed a well organised social system and communication, with division of labour among castes for specific duties. Other insects having advanced social life are ants, honey bees and wasps.


Lobatocerebrum and Jennaria (Rhynchocoela) are marine worms that are intermdediate between flatworms and annelida.

Washington giant earthworm (Driloleirus americanus) is one meter long; the Australian giant earthworm (Megascolides australis) is 3 meter long and the South African Microchaetus rappi is 20 feet long earthworm.

Clamworms feed by everting out their entire pharynx which has jaws at the bottom.

Earthworms burrow in mud by producing hydraulic skeleton by pumping coelomic fluid that makes the anterior region stone hard for burrowing.

Amazon leech (Haementaria) is 30 cm long. Leeches do not have true blood vascular system.

Leech has ten eyes but cannot see properly and finds the host by smell.

Some annelids are parasitic on other animals, such as Ichthyotomus is a parasite of fishes and Histriobdella is a parasite in the gill chamber of crustaceans.


Each gravid segment of tape worm carries about one hundred thousand eggs and hundreds of these proglottids are produced each day.

Hydated cyst of the dog tapeworm (Echinococcus) can grow to the size of a football in human tissues and it carries poisonous fluid.

Round worms (Nematodes) grow by enlargement of cells and not by multiplication of cells. Mitosis stops in nematodes after they hatch from eggs.

A single Ascaris can lay about 200,000 eggs daily and about 30 million in its lifetime.

Roundworm and hookworm larvae take a tour of all body organs before they reach intestine to become adult parasites.


Palytoxin obtained from the Hawaiian cnidarian, Palythoa toxica is more toxic than batrachotoxin of dart frog and is used to smear arrow tips.

Sea wasps (Cubozoa) are like small jelly fishes but swim very fast and their sting can kill a man in 15 minutes.

Hydra cannot digest starch.

Jellyfishes are named so because of the presence of enormous jelly-like mesogloea in their bodies.

The north Atlantic sea blubber, Cyanea capillata, is the largest jelly fish with a diameter of two metres.

Corals cannot grow beyond a depth of 50 metres in the sea.

The great barrier reef of Australia is over 1200 miles long and 70 miles wide.

Most of the coral reefs are in the Indo-Pacific region.

Sponges possess such extraordinary power of regeneration that even if they are crushed, mashed and strained through a cloth, the cells still rearrange themselves to form a complete sponge.

In asexually reproducing species, offspring always have more deleterious mutations than the parents. This is called Muller’s Ratchet.