Invertibrate Zoology Different Taxa Relationships

Arthropads

By nearly any measure, the most successful animals on the planet are the arthropods. They have conquered land, sea and air, and make up over three-fourths of all currently known living and fossil organisms, or over one million species in all. Since many arthropod species remain undocumented or undiscovered, especially in tropical rain forests, the true number of living arthropod species is probably in the tens of millions. One recent conservative estimate puts the number of arthropod species in tropical forests at 6 to 9 million species (Collins: 280-293)

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Arthropods range in distribution from the deep sea to mountain peaks, in size from the king crab with its 12-foot armspan to microscopic insects and crustaceans, and in taste from chocolate covered ants to crawfish jambalaya and lobster Newburg. Despite this unbelievable diversity, the basic body plan of arthropods is fairly constant. Arthropods have a stiff cuticle made largely of chitin and proteins, forming an exoskeleton that may or may not be further stiffened with calcium carbonate. They have segmented bodies and show various patterns of segment fusion (tagmosis) to form integrated units (heads, abdomens, and so on). The phylum takes its name from its distinctive jointed appendages, which may be modified in a number of ways to form antennae, mouthparts, and reproductive organs. (Aguinaldo: 489-493)

The appearance of the long-legged sea spiders are very similar to the "true", terrestrial spiders. There are usually four, but some species have six pair of legs. The body is slim. A respiratory system is not necessary, due to the relatively large body surface and small volume. The required oxygen diffuses through the surface. At the back of the head there is one or two pair of eyes. One pair is usually directed forwards, the other backwards. Some deep water species may reach a length of 90 cm, but many species are in the range of 3 to 30 mm. (Cotton: 49)

The sea spiders are marine arthropods found in all oceans. There are over 1300 registered species worldwide, 39 of them are known from the coast of Norway. The most common family Nymphonidae, is represented by 13 species in Norway. They are not "true" spiders, but are regarded as members of the same subphylum as the terrestrial spiders, in contrast to other arthropod groups, like the crustaceans. (Budd: 332-342)

Annelids

There are about 9,000 species of Annelids known today, including worms and leeches. They can be ...