Arthropods
Arthropoda The phylum Arthropoda, being the largest phylum in all of the kingdom Animalia, makes up about 84% of all known animal species. From tiny terrestrial insects to the aquatic king crab equipped with a 12 foot arm span, arthropods are evident in every habitat on Earth. Despite the diversified phylum, all members share a basic body structure containing a jointed skeletal covering with jointed appendages. An arthropod’s unique jointed exoskeleton provides support as well as locomotion with the help of the muscle system. Since they have an exoskeleton, arthropods lack locomotive cilia. The exoskeleton is composed of a epicuticle, a thin outer protein layer, and a procuticle, a thick chitin–protein layer. By the process of molting or ecdysis, arthropods shed off the old exoskeleton periodically to allow for growth. The underlying cells start by releasing enzymes to digest the base of much of the endocuticle and then the epidermis secretes a new skeleton beneath the old one. The old exoskeleton then splits along a specified line and the organism comes out of the old covering. Arthropods are triploblastic, having three germ cell layers: the ectoderm, endoderm and mesoderm. They have segmented bodies that bear jointed appendages. These jointed appendages give arthropods their name which means “jointed feet”. Arthropods exhibit bilateral symmetry much like humans. The right half of their bodies are mirror images of the left half. Contributing to the diverse life forms in Arthropoda are extremely diverse systems of feeding and digestion. Carnivores, herbivores, and parasites all exist within arthropods. The appendages at the mouth usually distinguish from arthropod to arthropod based on their diets. Spiders have a pair of fangs or chelicerae that deliver poison into their prey. Scorpions inject poison by a single stinger at the tip of their tails. Some insects have piercing appendages at the mouth to gnaw at vegetation and suck out plant juice.
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