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| Distinctive characters of Arachinda |
By:
Snigdha |
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The Arachnida (Gr. arachne, spider; oid, like), is a heterogeneous group of spider-like animals which differ markedly from one another. However, they resemble in several important respects.
I. They are generally small, air-breathing, mostly terrestrial and solitary arthropods, which are especially abundant in the warm dry regions.
2 The great majority are free-living, predaceous or parasitic.
3. The body is composed of 2 so mites, usually distinguished into .an anterior part, the prosoma or cephalothorax, and a posterior part, the opisthosoma or abdomen.
4. The cephalothorax bears simple and sessile eyes and six pairs of jointed limbs comprisirg one pair of chelicerae, one pair of pedipalpi and 4 pairs of walking legs The antennae, and the true jaws are entirely absent. The abdomen is usually limbless but sometimes en ds in a telson.
5. A plate-like internal skeleton, called the endosternite, is often present.
6. The respiratory organs are varied,. They are tracheae or booklungs or both in the terrestrial forms, In the aquatic forms respiration may be cutareous or taking place by book-gills.
7. The heart is dorsal, elongated and tubular as in other arthropods.
8. The excretory system comprises paired Malpighian tubules, or coxaiglands or both.
9. The rervous system may be concentrated anteriorly and it consists of a dorsal brain connected to a ventral nerve cord bearing paired garglia.
10. The sexes are mostly separate and the sexual dimorphism often inconspicuous. The genital aperture is single, opening mid-ventrally on the first abdominal segment, while the anus on the last segment.
11. Fertilization is usually internal, and development mostly direct, that is without a larval stage.
12. The arachnids are oviparous, but a few them are viviparous. |
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