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The Arthropoda is the most highly developed phylum of the invertebrates, and they contain the greatest number of species. Included are such widely recognized groups as the insects, spiders, crabs and barnacles.
General characteristics are a segmentation that varies in different parts of the body, often referred to as heteronomous metamerism. There is a chitinous exoskeleton, which is flexible but horny and tough. It is always mixed with other materials such as proteins and fats. They all possess jointed appendages, one per segment, which are primitive and never fully functional. The body cavity is a haemocoel that is not a true coelom. There are a dorsal brain, ventral ganglia and nerve cord, and there is more nervous tissue in the head than in the Annelida. They have three body regions, which may be combined. The digestive tract is always of three parts: (1) foregut, (2) mid gut and (3) hind gut. The foregut and hindgut are of ectoderm derivation while the midgut is derived from mesoderm. Molting is required as the cuticle is a non-growing part.
The Subphylum: Trilobitomorpha: Class: Trilobita – are an extinct group since the Paleozoic that is studied because they were probably the most primitive of the Arthropoda. Their body had a central region with a 3-lobed lateral area on either side. They had a head and trunk and were flattened dorso-ventrally. Compound eyes were present and one pair of appendages occurred on every segment of the body save the first. Superficially they bore a close resemblance to the Branchiopoda and also to the Brachyura crabs.
Trilobites were marine organisms and were numerous in the Cambrian and Silurian eras. They were mostly quite small at about 5 centimeters, but one fossil of Terataspis sp. reached about 0.7 meters in length. The anterior somites of a segmented trunk were movable on one another, while the most posterior segments were united to form a tagma known as the pygidium. Their body could be rolled up like that of an Isopod. It is thought that most species lived at the bottom of the ocean in shallow to moderately deep water. However some species are believed to have been adapted to pelagic, burrowing and deep-sea environments.
It is known that trilobites hatched as a larva, called the Protospis that was a bit circular and consisted mostly of a head. See Inv136 for example.
Subphylum: Myriapoda, Class: Chilopoda includes the centipedes. They are dorso-ventrally flattened. Their body consists of a head and trunk but there is no thorax nor abdomen. The head bears one pair of antennae, one pair of mandibles, one pair of maxillipedes with poison glands at the bases and ducts leading to pointed tips (Note: these are absent in the Diplopoda). There are two pairs of simple eyes called pseudocompound eyes. They have maxillae on the 1st and 2nd segments. The trunk bears uniramous appendages and there are 15 to 175 segments. See examples at Inv141.
Body Wall -- This consists of a cuticle, muscles and a haemocoel
Digestive Tract -- A typical mouth to anus arrangement.
Circulatory System -- The heart is tubular with one pair of ostia per segment. The blood does not carry oxygen
Respiration -- The tracheae are lined with ectoderm and cuticle, and heavy rings of cuticle line them. They branch out and ultimately reach all tissues of the body. The blood does not have an oxygen carrying function.
Excretion -- Malpighian tubules are long, thread-like and blind-ending tubules. They lie in the haemocoel and empty into the digestive tract at the junction of the mid and hindguts. They extract nitrogenous wastes from the blood.
Nervous System -- This system is the same as that found in the Crustacea.
Reproduction -- The sexes are separate. Genital organs are found at the posterior end of the body and development is direct.
Locomotion -- These animals are fast movers. Long posterior legs are sensory and used when moving backwards.
Food & Digestion -- Chilopoda are carnivorous and their food is paralyzed first by the maxillipedes.
Subphylum: Myriopoda, Class: Diplopoda includes the millipedes. These are cylindrical animals with a head and trunk that is the same as in the Chilopoda. The head appendages include antennae, mandibles, one pair of maxillae (instead of 2 pair as in the Chilopoda) and pseudocompound eyes on the head. The trunk has 25-100 or more segments with each segment bearing two pair of appendages. A fusion occurs between two segments all along the body except on the first trunk segment. See example at Inv142.
Reproduction -- Genital ducts open between the bases of the 2nd and 3rd legs. This is anterior in contrast to the Chilopoda.
Development -- it is essentially direct except that the first larva has only three pairs of legs with additions added later on.
Food -- They are vegetarian animals that move very slowly and are harmless.
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