Phylum Annelida is a group of aquatic and terrestrial worms with segmented bodies. The worms have various segments with the main parts being a head with a mouth, complete digestive tract, and an anus at the tail end of the worm. Each segment has nervous, circulatory, and excretory systems. In addition, the bodies have longitudinal and circular muscle fibers with an outer moist, acellular cuticle. The phylum has major classes, which are Polychaeta, Hirudinea, and Oligochaeta. The common members of this phylum are earthworms and leeches, which belong to class Oligochaeta and Hirudinea respectively. Oligochaetes inhabit freshwater and terrestrial habitats while leeches are found in freshwater, terrestrial and marine habitats. The less common members are sandworms, clamworms, and tubeworms, which belong to class Polychaeta that inhabits marine ecosystems.
Phylum Arthropoda is the most successful group of animals found on the Earth because they have conquered the sea, air, and land. Their bodies have an exoskeleton made of chitin and proteins, which is periodically shed or molted during growth and development. They are also segmented into three main parts, which are head, thorax and abdomen. In addition, they have jointed legs and appendages that are attached to the exoskeleton that form reproductive organs, antennae, and mouthparts. The phylum has three major classes, which are Crustacea, Chelicerata, and Insecta. Chelicerata includes spiders, ticks, mites, scorpions, and daddy-long-legs. These animals have a fused thorax and head to form a cephalothorax with four pairs of legs extending from it. They have two pairs of appendages on their head: chelicerae and pedipalps. A majority of animals in this class is blind, but some can have up to eight eyes. Class Insecta is the most diverse with a common body plan of head, thorax and abdomen. They also have three pairs of jointed legs extending from the thorax, a pair of sensory antennae on the head, and a pair of wings. The third class is Crustacea, which includes lobsters, crabs, and shrimps. They have three body parts comprising of head, thorax and abdomen, and two pairs of sensory antennae projecting from the head. The thorax has five pairs of legs extending from it, and a shell called carapace that covers both the thorax and head. The carapace also protects a majority of the other organs of crustaceans.
Evolution has played a significant role in the development of modern Annelids and Arthropods. It is thought that Annelids and Arthropods originated from a common polychaete ancestry about 500-600 million years ago. The first animals had soft bodies, multiple segments, and bilateral symmetry. They lacked a distinct head, eyes or antennae. The worms had similar characteristics with arthropods including an open circulatory system, clawed pairs of legs, antennae, paired appendages, and trachea for respiration. The first arthropods resembled worms in appearance and structure, but differed slightly in that they had an exoskeleton, which is their hallmark. The arthropods evolved to modern ones by developing segmented appendages, chitinous exoskeleton, striated muscles, open circulatory system, dorsal heart, and well-developed mouthparts and head. Additionally, Arthropod eggs are encased in a large proteinaceous shell unlike those of the ancestral annelids.
Metamerism is a distinct feature of both annelids and arthropods because it involves the segmentation of the body into many parts called metameres or somites. There is a serial repetition of homologous body parts such as appendages, nerves, and muscles among others, but the organs function in accord with other body parts. Newly formed parts are added onto the posterior part of the body called pygidium while the anterior part is called acron. In arthropods, metamerism leads to the segmentation of the body into head, thorax and abdomen. Metamerism allows easy muscle contraction, increasing the efficiency of body movement. It also facilitates the development of higher complexity in general body organization.
Serial homology is the repetition of body structures in one organism. This occurs in both vertebrates and invertebrates where in invertebrates it is exemplified by jointed appendages. For example, in a crayfish, the reproductive organs, antennae, mouthparts, and legs are similar to the structures it uses to swim backwards. All the structures follow a similar pattern, which is a perfect example of serial homology. Conversely, tagmatization is the adjoining of distinct body parts of animals particularly arthropods into larger functional units called dogma. These units have specialized tasks in the animal’s body, for example, the fusion of head and thorax to form cephalothorax in class Chelicerata.
There are genes that are homologous to all animals and play a significant role in the development of major body parts. These are Hox genes or control genes that help in the layout plan of the body from head to tail during early development. In arthropods and other vertebrates, they direct the formation of appendages and other body parts along the anterior-posterior axis. In addition, they also regulate maturation of cells for specific functions, also known as differentiation. Hox genes are described as general-purpose genes because they are homologous in a majority of organisms. The same genes are found in both vertebrates and invertebrates to direct development of body parts.
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