9.3.8 Economic Importance of Arthropods
They are both useful as well as harmful to mankind.
Crustaceans:
Provide food directly and indirectly to humans (such as prawn, crab, lobster, etc. Some are harmful because act as as intermediate hosts for human parasites (larvae of nematodes) carried by Cyclops,
Beneficial insects:
Help in pollination (such as ants, butterflies, and bees). Used as food in some parts of the world (such as grasshoppers, and cricket). Scavengers; Eat dead and decaying plants and animals. Commercial substances such as honey, bee's wax, silk, and shellac are produced by the honey bees, silkworms, and lac insects respectively.
Destroy useless weeds by feeding upon them. Eat other harmful insects such as dragonflies feed on mosquitoes.
Scientific use:
Several insects are being used for scientific studies, such as cockroaches, fruit flies, grasshoppers, etc.
Harmful insects:
Destroy stored foods and grains. (ants and weevils).Carrier of many parasitic diseases (mosquito; malaria), (Tsetse fly; sleeping sickness). Damage crops, fruit trees, and timber trees: For example grasshoppers, bugs, locusts, beetles, caterpillars, weevils, aphids, etc.
Damage books (silverfish), and household articles (such as white ants destroy furniture). They irritate humans in various ways, such as bees sting, and cause many eye diseases. There are certain blood-sucking insects e.g., Louse.
Arachnida:
Poisonous arachnids like scorpions and certain spiders sting humans. Ticks and mites are parasitic disease carriers. Mites destroy crops. Beneficial arachnids like scorpions and spiders feed on injurious insects. 9.3.9 Phylum Echinodermata (Spiny skinned animals) (Greek: Echinos means spine, derma means skin)
Habit:
Mostly free living, some are attached to a substratum. Phylum's name was given by Leuckert in 1847.
Numbers:
More than 5,000 known species have been so far recorded.
Body:
a) Triploblastic,
b) Coelomate.
c) Adult is radially symmetrical, d) A delicate epidermis covers the body, under which there is a firm mesodermal calcareous exoskeleton. e) The mouth is on the lower surface (oral) and the anus is on the upper surface (aboral). f) From a central disc arms are radiated.
Shape:
Biscuit shape, (such as cake urchin), star-shaped with short arms (starfish). globular (sea urchin ), star-shaped with long arms (brittle star), or elongated (sea cucumber).
Water vascular systems: It is a system of complex tubes and spaces surrounding the mouth and passing into the arms and tube feet. In echinoderms, water circulates through these coelomic channels, which enter through a sieve-like plate called madreporite (present in the aboral body surface).
Locomotion occurs through tube feet. Digestive and reproductive systems are well developed. No specialized organs for respiration or excretion. A poorly developed nervous system consists of only a pharyngeal nerve
The circulatory system is also less developed. Fertilization is external and these are unisexual. Development is indirect (complex and bilaterally Bipinnaria and Brachiolaria. Regeneration is common.
Examples:
Starfish, sea cucumber, sea Lilly, brittle stars, and sea urchins. Evolutionary Adaptations in Echinoderms: larvae named Echinoderm are the first and only invertebrates that are deuterostomes, therefore, these are placed at the top of invertebrates, near chordates.
Their body structure is simple, exclusively marine, (either benthic or pelagic). Digestive System usually complete, axial, or coiled anus, absent in group ophiuroids. The nervous system is reduced due to marine nature, no brain, only a nerve ring and radial nerve cord are present. Breathe by dermal gills, tube feet, and respiratory tree. No excretory organs.
Economic importance of echinoderms:
Many are used as food, and starfish act as scavengers, and thus clean seawater. Many echinoderms are used as fertilizer because their dried skeleton contains large amounts of calcium and nitrogenous compounds. Many echinoderms are poisonous such as sea urchins, sting humans, and other animals. They also damage oyster beds.
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