Introduction:

Approx 75-80% of all known animals are arthropods; they comprise largest phylum in animal kingdom. Arthropods are found at all depths in brackish, marine, and fresh waters. They are common in temporary ponds, hot springs, saline lakes, and underground rivers. On dry land they are located in deserts, forests, grasslands and tundra. Their success as terrestrial animals possibly owes lot to their water-conserving excretory system, gaseous exchange organs and desiccation-resistant impermeable cuticle. Arthropods have segmented bodies having two or more distinct regions known as tagma. Body is enclosed by the tough, at times flexible, chitinous exoskeleton which serves for both protection and attachment of muscles and other soft tissues. This covering is simple device which may describe part of amazing success of group. Discarded casts were frequently preserved as fossils.

The Exoskeleton of arthropods:

The main distinguishing feature of arthropods, immediately visible to the casual observer, is a tough, semi-rigid cuticular exoskeleton secreted by the epidermis and consisting basically of chitin, a polysaccharide, enmeshed with the protein arthropodin. The cuticle is a layered structure, with a thin, non-wettable outer surface. Although the cuticle is always built on a foundation of chitin, this may be invaded or overlaid by different materials in the various arthropod groups, as we mentioned in the 'Introduction'. In insects, the cuticle is rather thin and light but is impregnated with tanned proteins that give it strength and made waterproof by an outer layer of wax. In forms like crabs the cuticle is thick and hard due to the deposition of calcium salts in the endocuticle; a degree of water-proofing is provided by lipids in the outer epicuticle, and the whole structure is protected by an outer layer of cement.

Categorization and features of Arthropoda:

Features of Phylum Arthropoda are given below:

• They are bilaterally symmetrical, segmented protostomes, basically prepared with the pair of appendages joined to each body segment. Appendages are though frequently reduced or lost in some of body segments.
• Though all post-oral appendages were similar in primitive forms, modern arthropods have appendages which have been specialized for specific functions. Much of arthropod evolution is reflected in specialization of appendages, and nature of appendages is significant factor in categorization.
• Surface of body and appendages is enclosed by continuous cuticle of complex structure. In many cases cuticle forms series of heavy, skeletal plates or rings, linked by thinner, flexible articular membranes, permitting freedom of movement.
• Heavy cuticle extends inwards to give spines or shelves for muscle attachment; extensions are known as apodemes.
• Cuticle forms chitinous linings of foregut and hindgut. Linings are molted with surface cuticle during growth.
• Contractile heart, derived from the dorsal blood vessel, lies in dorsal pericardial sinus. Blood enters heart through pairs of apertures called as ostia (sin. ostium).

Subphylum Trilobitomorpha primitive arthropods), Trlobites:

The extinct group of arthropods signified by fossils in which body was molded longitudinally in three lobes, therefore the name Trilobitomorpha. They had pair of antennae and all appendages on post-antennal somites were of common type. They were marine arthropods and were extremely abundant in Cambrian and Silurian but became extinct by secondary period. E.g. Olenus etc.

Subphylum Chelicerata (Greek: chele, talon; cerata, horns)

There are approx 63,000 described species of Chelicerata. Subphylum represents one of the major arthropod evolutionary lines and it comprises well-known spiders, scorpions and ticks. Chelicerates do not have antennae and first pair of appendages (chelicerae), are utilized in feeding; name of subphylum is derived from chelicerae. Body is separated in two main regions, prosoma, composed of head and thorax, and opisthosoma, which is abdomen.

Features of Chelicerata are given below:

• Bilaterally symmetrical, less than 1 mm-60 cm long arthropods varying in body shape from elongate to almost spherical.
• They are triploblastic, having tissues and organs.
• A through, straight gut, from mid-gut region of which issue from two to several pairs of digestive diverticula (branches), that secrete enzymes and intracellularly digest and absorb food; mouth is anteroventral.
• Body divided in 2 regions, anterior 'prosoma' formed by acron and 6 appendage- bearing segments, and completely or partially covered by dorsal carapace, and posterior opisthosoma without legs and with only highly altered appendages, if any.
• Prosomal excretory system of blind-ending coxal glands, and/or opisthosomal system of branched endodermal Malpighian tubules arising from mid-gut and discharging mainly guanine.
• A non-calcareous exoskeleton and sometimes also with a plate-like mesodermal endoskeleton in the prosoma.
• Blood system engaged in circulation of respiratory gases and generally having haemocyanin.
• Nervous system with separate ganglia along length of body or, more generally, concentrated into single prosomal mass.
• Gonochoristic (dioecious), with external fertilization in exclusively marine classes (though two partners relate closely in pseudocopulation during mating) and internal fertilization by copulation or spermatophores in mainly terrestrial class; gonopores are on second opisthosomal segment.
• Originally benthic marine, one class contains colonized land and freshwater very successfully.

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