Most chelicerates have four pairs of walking legs Plasma Membrane

Most chelicerates have four pairs of walking legs

Fri, 27 Jan 2012 13:34:57 | Plasma Membrane

In the body plan of chelicerates (phylum Chelicerata), the anterior region (head) bears two pairs of appendages modified to form mouthparts. In addition, many chelicerates have four pairs of walking legs. The 63,000 described chelicerate species are usually placed in three classes: Pycnogonida, Merostom-ata, and Arachnida; most of them belong to Arachnida.

The pycnogonids (class Pycnogonida), or sea spiders, are a poorly known group of about 1,000 marine species (Figure 33.16a). Most are small, with leg spans less than 1 cm, but some deep-sea species have leg spans up to 60 cm. A few py-

33.15 Myriapods

(a) Centipedes have powerful jaws for capturing active prey.

(b) Millipedes, which are scavengers and plant eaters, have smaller jaws and legs.They have two pairs of legs per segment, in contrast to the one pair on each segment of centipedes.

(a) Scolopendra heros

(a) Scolopendra heros cnogonids feed on algae, but most are carnivorous, feeding on a variety of small invertebrates.

The class Merostomata contains the horseshoe crabs (order Xiphosura), with five living species, and the extinct giant water scorpions (order Eurypterida). Horseshoe crabs, which have changed very little during their long fossil history, have a large horseshoe-shaped covering over most of the body. They are common in shallow waters along the eastern coasts of North America and Southeast Asia, where they scavenge and prey on bottom-dwelling invertebrates. Periodically they crawl into the intertidal zone in large numbers to mate and lay eggs (Figure 33.16b).

Arachnids (class Arachnida) are abundant in terrestrial environments. Most arachnids have a simple life cycle in which miniature adults hatch from internally fertilized eggs and begin independent lives almost immediately. Some

33.15 Myriapods

(a) Centipedes have powerful jaws for capturing active prey.

(b) Millipedes, which are scavengers and plant eaters, have smaller jaws and legs.They have two pairs of legs per segment, in contrast to the one pair on each segment of centipedes.

arachnids retain their eggs during development and give birth to live young.

The most species-rich and abundant arachnids are the spiders, scorpions, harvestmen, mites, and ticks (Figure 33.17). The 30,000 described species of mites and ticks live in soil, leaf litter, mosses, and lichens, under bark, and as parasites of plants, invertebrates, and vertebrates. They are vectors for wheat and rye mosaic viruses, and they cause mange in domestic animals and skin irritation in humans.

Spiders are important terrestrial predators. Some have excellent vision that enables them to chase and seize their prey. Others spin elaborate webs made of protein threads in which they snare prey. The threads are produced by modified abdominal appendages connected to internal glands that secrete the proteins, which dry on contact with air. The webs of different groups of spiders are strikingly varied, and this variation enables the spiders to position their snares in many different environments. Spiders also use protein threads to construct safety lines during climbing and as homes, mating

: (a) Decalopoda sp.

33.16 Minor Chelicerate Phyla (a) Although they are not spiders, it is easy to see why sea spiders were given their common name.

(b) This spawning aggregation of horseshoe crabs was photographed on a sandy beach in Delaware.

: (b) Limulus polyphemus

: (b) Pseudouroctonus minimus

(d) Brevipalpus phoenicis

33.17 Arachnid Diversity (a) The blackjumping spider's bite produces an inflammatory reaction on mammalian skin. (b) Scorpions are nocturnal predators. (c) Harvestmen, also called daddy longlegs, are scavengers. (d) Mites are blood-sucking, external parasites on vertebrates.

(d) Brevipalpus phoenicis

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