Phylum Arthropoda

The largest phylum in the animal kingdom, arthropods are segmented animals with paired joined legs, a hard joined exoskeleton, a complete digestive tract, reduced coelom, no nephridia, a dorsal brain, and a ventral nerve cord with paired ganglia in each segment.

There are almost 1,000,000 known species of them on the Earth. There are five main groups of arthropods: insect, millipede, centipede, crustacean, arachnid.

Arthropods do not have bones inside their bodies. Instead, arthropods have a hard outer covering – an exoskeleton – on their bodies. The exoskeleton gives protection, provides a place where muscles attach, and allows movement. It is made up of nonliving material. This material cannot grow as the rest of the animal grows. When an arthropod’s skeleton becomes too small for the animal inside, the exoskeleton must be shed. The shedding process is called molting. A new exoskeleton has already begun to grow underneath. The new exoskeleton is soft and moist and provides little protection. Many arthropods hide after they molt until their new exoskeleton becomes hard.

5.8.1. Class Crustacea

Crustaceans are mostly aquatic, with two pairs of antennae, one pair of mandibles, chewing jaws and typically two pairs of maxillae. The thoracic segments have appendages, and the abdominal segments are with or without appendages. About 25,000 species.

The crustaceans include crabs, crayfish, lobsters, barnacles, shrimps, Daphnia (water fleas), and a number of smaller forms found mostly in fresh water and marine plankton, as well as some terrestrial forms. Crustaceans are mandibulates. Their body is divided into 2 main parts – a cephalothorax and an abdomen.

Unlike other arthropod groups, almost all the crustaceans are aquatic, but some, crabs in particular, are amphibious or land dwellers. The crustaceans can live in oceans and along the seashores (many of these crustaceans are highly prized as food); other crustaceans, such as crayfish and water fleas, live in lakes and ponds. A few crustaceans live in moist soil on land.

Amphibious crabs continue to breathe with gills, in their thoracic cavities which keep the gills wet and aerating the water through holes in their exoskeletons. The true land crab has lost the gill structures and instead has an area of highly vascularized epithelial tissue through which oxygen is exchanged.

The crayfish is a typical crustacean. It has the upper body region called a cephalothorax. The lower body region is its abdomen. Jointed legs are attached to the cephalothorax. The front pair of legs with the large claws are known as chelipeds. They are used for protection and catching the food and not for walking. The jaws the crayfish uses to chew its food are mandibles. Leglike maxillipeds hold the food in place. The crayfish uses its antennae to taste, smell and touch its surroundings. The small feathery structures under the crayfish’s abdomen are the swimmerets. They are used for forcing water over the gills and for reproducing. The tail of the crayfish is the flipper. When the flipper is moved down and forward, the crayfish lurches backward. The lurching motion usually stirs up a protective muddy cover that hides the crayfish from its enemies.

Respiration

The crayfish moves oxygen from the water by means of its gills. The featherlike gills contain a high concentration of blood vessels. The gills are attached to the walking legs, between the thorax and the carapace. As the animal walks, water flows over the gills.

Circulation

Crayfish, like spiders, have an open circulatory system. Blood enters the heart through three pairs of pores called ostia. Valves seal off the ostia, the heart contracts, and blood is forced into seven large arteries. These arteries then discharge blood into the spaces surrounding the organs. The blood drains out of these spaces and collects in a cavity called the sternal sinus. From there blood travels through other vessels to the gills. Blood returning from the gills enters the pericardial sinus and then returns to the heart through the ostia.

Digestion and Excretion

Crayfish eats living and dead plants, worms, larvae and tadpoles. Food moves from the mouth down the esophagus to the stomach, where it is ground by teeth of chitin. Undigested wastes move through the intestine and leave the body through the anus. Excretory organs called green glands are located on the base of the antennae. The green glands remove liquid wastes from the blood.

Nervous System

The nervous system of crayfish consists of a dorsal brain formed from a pair of ganglia. Branches from the brain run to the eyes, antennules and antennae. Other nerves encircle the esophagus and connect with a ventral nerve cord that runs the length of the body. Each of the crayfish’s segments has a pair of ganglia. The crayfish has two eyes located on movable stalks on the front of the body. Each eye has over 2500 lenses. Eyes that have more than one lens are called compound eyes. Compound eyes form images by combining the sensations from multiple lenses. Such eyes respond rapidly to light and readily detect motion. Arthropods are the only animals with compounds eyes.

Reproduction and Growth

Crayfish mate in the spring or fall. The male deposits sperm into the female, who stores it. About two weeks later, the female produces 200 to 300 eggs. They are then fertilized by the stored sperm and hatch in six weeks. The fertilized eggs are attached to the last three pairs of swimmerets of the female until they hatch. Crayfish molt twice a year. Molting begins when the outer layer of body cells digests the inner layer of the exoskeleton weakening it. The crayfish swells by absorbing water and taking in excess air. This causes the exoskeleton to crack and the crayfish backs out. The outer body cell layer secretes salts that harden the new, developing exoskeleton.

5.8.2. Class Arachnida

Arachnids include spiders, mites, scorpions, ticks, daddy long-legs. Most members are terrestrial, air-breathing; usually have 4 pairs of legs. The arachnids are chelicerates. The first pair of appendages are chelicerae, which are sharp and pointed and are used for capturing and then paralyzing prey by the injection of a poison. The second pair are pedipalps, which are used for handling and tearing food. There are about 30,000 species.

Most arachnids live on land. Many ticks and mites can be found on the skin of animals and people. Arachnids, like the crustaceans, have two body parts: a cephalothorax and an abdomen. Arachnids also have eight jointed walking legs.

Spiders, the most familiar arachnids, are among the word’s most fascinating animals. But very few of the 30.000 species are harmful to humans. Spiders are contribute to human welfare by eating insect pests.

Feeding

Spiders, like other arachnids, live on a completely liquid diet. Spiders feed by injecting enzymes into their prey to dissolve the body substances. The spiders then suck out and swallow the liquefied remains. Malpighian tubules found near the base of the abdomen form the excretory system of the spider. The Malpighian tubules remove excess nitrogen, a by-product of metabolism, from the blood. Spiders also have waste-removing organs on the first and third pairs of legs.

Respiration

Oxygen is carried through the spider’s body in two unusual ways. One way is through tubes called tracheae, which carry air directly to the cells. The tubes receive air through slits in the exoskeleton called spiracles. The opening and closing of the spiracles regulates air flow. In the second method of respiration, blood circulates through a structure called the book lung, so named because its sheets of tissue hang down like the pages of the book. The book lung is located near the front of the abdomen and absorbs oxygen through the spiracles. Some spiders have either tracheae or book lungs but most spiders have both. These structures are unique to arthropods.

Reproduction

Like most arthropods, spiders are either male or female. A male spider approaches a female with caution. Since spiders are habitually solitary animals, a female might mistake a male suitor for potential prey. For this reason many species engage in complex courtship rituals, such as taping on the web or stroking the female. Having captured the attention of the female, the male puts sperm on his pedipalps and puts it into her genital opening on the outside of the abdomen. Eggs, laid in special webs or cocoons, hatch in about two weeks.

The weaving of a web is not learned behavior, nor does it require any practice. A spider confined from egg to adult can weave a perfect web on its first try. A spider releases silk for its web through openings in its abdomen. On the posterior portion of the spider’s abdominal surface is a cluster of spinnerets, fingerlike organs from which a fluid protein exudes that hardens into silk as it comes into contact with the air. Spiders spend a great deal of energy rebuilding their webs every day. Some webs can be as large as a meter around. Webs trap insects and other small animals that the spiders use for food. Silk is used not only for the variety of webs made by the different species but for a number of other purposes as well, such as for a drop line, on which the spider goes sky diving, for a cocoon, for lining a burrow, for the shroud of a victim or for wrapping an edible offering presented to the female of certain species by the courting male. Most spiders can spin several kinds and thicknesses of silk.

Scorpions

Scorpions have two distinctive features. Their greatly enlarged pedipalps are held in a forward position, and they have an abdomen that ends in a tapered stinger. Scorpions are most common in tropical areas and deserts. The 800 known species range in length from 1.3 to 17.6 cm. Scorpions hide under rocks and in crevices by day and are active mainly at night. They feed on insects and spiders. A scorpion catches and holds its prey in its pedipalps until the stinging abdomen can curl over the top of its body and inject poison into the prey.

Ticks and Mites

Ticks and Mites are tiny animals, usually less then 1 mm long. They differ from other arachnids in that the cephalothorax and abdomen are fused to form a single body part. parasites. Ticks attach themselves to an animal’s skin and suck blood. Many ticks carry disease-causing organisms that are ransmitted to other animals through the tick’s bite. Most human beings have mites living in their hair follicles. The mites live off.

Date: 2014-12-22; view: 948