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Chapter

BIO2231: Phylum Arthropods characteristics

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Department
Biology
Course
BIO2231
Professor
Various
Semester
Spring

Description
PHYLUM ARTHROPODS Characteristics of Phylum Arthropoda: o Jointed appendages; ancestrally, one pair to each segment, but number often reduced; appendages often modified for specialized functions o Living in marine, freshwater, and terrestrial habitats; many capable of flight o Free-living and parasitic taxa o Bilaterial symmetry; segmented body divided into functional groups called tagmata: head and trunk; head, thorax, and abdomen; definite head o Tripoblastic body o Reduced coelom in adult; most of body cavity consisting of hemocoel (sinuses, or spaces, in the tissues) filled with blood o Cuticular exoskeleton; containing protein, lipid, chitin, and often calcium carbonate secreted by underlying epidermis and shed (molted) at intervals; although chitin occurs in a few groups other than arthropods, its use better developed in arthropods o Complete digestive system; mouthparts modified from ancestral appendages and adapted for different methods of feeding; alimentary canal shows great specialization by having, in various arthropods, chitinous teeth, compartments, and gastric ossicles o Complex muscular system, with exoskeleton for attachment, striated muscles for rapid actions, smooth muscles for visceral organs; no cilia o Nervous system similar to that of annelids, with dorsal brain connected by a ring around the gullet to a double nerve chain of ventral ganglia; fusion of ganglia in some species o Well-developed sensory organs; behavioral patterns much more complex than those of most invertebrates; oviparous, ovoviviparous, or viviparous; often with metamorphosis o Paired excretory glands called coxal, antennal, or maxillary glands present in some; others with excretory organs called Malpighian tubules o Respiration by body surface, gills, tracheae (air tubes), or book lungs o Open circulatory system, with dorsal contractile heart, arteries, and hemocoel (blood sinuses) Phylum Arthropoda: Arthropods are ecdysozoan protostomes. They have segmented bodies, a chitinous cuticle often containing calcium, and jointed appendages. The critical stiffening of the cuticle to form a jointed exoskeleton is sometimes called ‘arthropodisation’. Arthropods diversified greatly, but it is relatively easy to identify particular body plans characterizing arthropod subgroups. For example, centipedes and millipedes have trunks composed of repeated similar segments, whereas spiders have two distinct body regions and lack repeated segments. Why Have Arthropods Achieved Such Great Diversity and Abundance? 1. A versatile exoskeleton. Arthropods possess an exoskeleton that is highly protective without sacrificing flexibility or mobility. This skeleton is the cuticle, an outer covering secreted by the underlying epidermis. The cuticle consists of an inner, relatively thick procuticle and an outer, relatively thin epicuticle. Both the procuticle and the epicuticle each consist of several layers (lamina). The outer epicuticle is composed of protein, often with lipids. The protein is stabilized and hardened by chemical cross-linking, called sclerotization, which increases its protective ability. In many insects the outermost layer of epicuticle is composed of waxes that reduce water loss. The procuticle is divided into an exocuticle, which is secreted before a molt, and an endocuticle, which is secreted after molting. Both of these layers are composed of chitin bound with p
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