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The Phylum Annelida Etymology:- From the Latin Annellus a little ring. Characteristics of Annelida:1)Bilaterally symmetrical and vermiform. 2)Body has more than two cell layers, tissues and organs. 3)Body cavity is a true coelom, often divided by internal septa. 4)Body possesses a through gut with mouth and anus. 5)Body possesses 3 separate sections, a prosomium, a trunk and a pygidium. 6)Has a nervous system with an anterior nerve ring , ganglia and a ventral nerve chord. 7)Has a true closed circulatory system. 8)Has no true respiratory organs. 9)Reproduction normally sexual and gonochoristic or hermaphoditic. 10)Feed a wide range of material. 11)Live in most environments 1 The Annelida are a medium sized phylum of more than 9,000 species of worms. Most species prefer aquatic environments, but there are also a number of well know terrestrial species. Only a few species of annelids are commonly known to human beings, these include Earthworms that work so hard to make our soils healthy, In many countries people are still familiar with Medicinal leeches, and people who live closer to nature are naturally more familiar with a much wider range of Annelids than those who live in cities. The Annelida The earthworms, of which there are many species, are exceedingly important in soil creation, particularly in temperate areas. Without them, agriculture and perhaps the whole of human society as we know it would never have evolved. Like so much of the unnoticed invertebrate world earthworms are essential to our very existence. In marine environments the numerous species of Polychaetes play a fundamentally important role in the maintenance of food chains and the whole ecological balance of the seas, thus supporting the seemingly endless stocks of fish we like to eat 2 The Annelida Annelids range in size from the Giant Earthworms, of which Michrochaetus rappi is the largest, this magnificent animal has an average length of 1.36 m (54 ins) and a record breaking specimen has been recorded that measured 6.7 metres (22 ft) in length. The smallest Annelid known to science is Chaetogaster annandalai which is full grown at 0.5 mm (0.02 ins). 3 Characteristics Annelids have two main modes of existence, they either live rather quietly in holes or they live more active lives. The basic Annelid body plan is one of a head followed by a long thin body of numerous similar segments ending in a small tail. The head consists of a mouth (prostomium) and sometimes a peristomium , and the tail is more correctly called a pygidium, as it is not really a tail. Annelids are coelomate animals meaning they have a true coelom within their body. They have sets chaetae attached to each body segment, and these can be simple and small as in the Earthworms or complex and varied as in many Polychaetes. The head is often reduced and difficult to distinguish in the hole living species, but may be easily recognised, with eyes and other sensory devices in those species living a more active life. coelomate animals Annelids are coelomate animals (meaning they have a true coelom, even if this is reduced secondarily). They normally have long thin bodies composed of a series of identical segments. These segments lie between the head, comprised of a prostomium, a mouth and sometimes a peristomium, and a tail called a pygidium. Growth occurs both laterally, by enlargement of the segments during the juvenile stages, and through the addition of new segments. New segments are produced by the foremost section of the pygidium. In some species they are produced throughout the animals life but in many species production stops once a certain set number of segments has been achieved. 4 The Phylum Annelida Is divided into 3 classes, one of which the Clitellata could really be called a Superclass, it contains three subclasses: The Oligochaeta the Branchiobdella The Hirundinea. a. The other two classes are the Polychaeta which contains the largest number of species a. The Aelosomatida which contains very few species Scientific or biological classification Class:Polychaeta Class; Aelosomatida class:Alitellata Subclasses Oligochaeta Branchiobdella Hirundinea 5 1-The class Polychaeta (Poly = many, Chaeta = bristle) are the most diverse and most speciose group of the Annelida containing over 5,500 species. They are predominantly marine animals and are divided ecologically into the Errantia and the Sedentaria It is useful as it divides the class in two , in terms of the number of families each group contains. The Errantia have well developed heads and complex parapodia (paddles)that they can use for swimming. They are often dorsoventrally flattened. 6 1-The class Polychaeta Most polychaetes are gonochoristic (meaning they are either male or female) Some are sequential hermaphrodites (meaning they are one sex first and then change to being the other sex). Reproduction is often accompanied by the production of special modified reproductive segments which may, or may not, become independent of the parent worm before mating. These segments are destroyed or die during or immediately after they have released their gametes (sperm and ova). 7 The class Polychaeta 2-The class Aelosomata contains about 25 species of small to minute worms with many chaetae. They live in the interstitial zone of both fresh and salty water environments. They are hermaphrodites with each animal possessing one ovary and two testis. 8 The class Clitellata They are normally hermaphrodites, and possess a clitellum as adults, an organ which looks like a bandage of skin wrapped around the animal. This clitellum, from which the whole group takes its name has an important function in sexual reproduction, otherwise reproduction may asexual by fission (division). 1-The subclass Oligochaeta (Oligo = few, Chaeta = bristle) are the second most numerous group of annelids with around 3,100 species. Oligochaeta live in marine, freshwater and terrestrial habitats. Generally they have a more rounded crosssection A less distinct head and are less diverse in form than the Polychaetes. 9 2-The subclass Branchiobdella contains 147 species of small (about 1 cm long) aquatic whitish animals that are either commensals or parasites on Crayfish. They are mostly found in the northern hemisphere. Different species attach to their hosts at different places on the body, thus Branchiobdella parastica attaches to the under side of the abdomen while Branchiobdella astaci attaches to its hosts gills. Branchiobdella hexodonta and B. astaci are known to be parasitic feeding off host tissue but B. parasitica is thought by some authorities to be a commensal 2-The subclass Branchiobdella contains 147 species of small (about 1 cm long) aquati whitish animals . that are either commensals or parasites on Crayfish. They are mostly found in the northern hemisphere. Different species attach to their hosts at different places on the body, thus Branchiobdella parastica attaches to the under side of the abdomen while Branchiobdella astaci attaches to its hosts gills. Branchiobdella hexodonta and B. astaci are known to be parasitic feeding off host tissue but B. parasitica is thought by some authorities to be a commensal 10 3-The subclass Hirundinea contains the 500 or so species of animals commonly known as leeches. Leaches are well known for their blood sucking habits and their head to tail looping mode of locomotion. Except for the primitive Acanthobdella peledina leeches have no chaetae and 33 body segments. they have two suckers which in most cases are located one at the anterior (head) end of the body composed of segments 1-4 and the other at the posterior (tail) end composed of segments 25-33. 11 Leaches Like the Oligochaeta from which they are believed to have evolved the Hirundinea occur in Fresh water, marine and terrestrial environments 12 Anatomy Septum Muscle layer segments 13 Cross section Anatomy Annelids are triploblastic protostomes with a coelom ,closed circulatory system, and true segmentation. Protosomes are animals with bilaterial symmetry where the first opening in development, the blastophore, becomes its mouth. Triploblastic means that they have three primary tissue areas formed during embryogenesis. A coelom is a fluid-filled body cavity. 14 For drawing Oligochaetes and polychaetes Oligochaetes and polychaetes typically have spacious coeloms; in leeches, the coelom is largely filled in with tissue and reduced to a system of narrow canals; archiannelids may lack the coelom entirely The coelom is divided into a sequence of compartments by walls called septa. In the most general forms, each compartment corresponds to a single segment of the body, which also includes a portion of the nervous and (closed) circulatory systems, allowing it to function relatively independently. Each segment is marked externally by one or more rings, called annuli. 15 Oligochaetes and polychaetes Each segment also has an outer layer of circular muscle underneath a thin cuticle and epidermis, and a system of longitudinal muscles. In earthworms, the longitudinal muscles are strengthened by collagenous lamellae; the leeches have a double layer of muscles between the outer circulars and inner longitudinals. In most forms, they also carry a varying number of bristles, called setae, and among the polychaetes a pair of appendages, called parapodia. 16 Digestive system 17 Oligochaetes and polychaetes Anterior to the true segments lies the prostomium and peristomium, which carries the mouth, and posterior to them lies the pygidium, where the anus is located. The digestive tract is quite variable but is usually specialized. For example, in some groups (notably most earthworms) it has a typhlosole (internal fold of the intestine or intestine inner wall), to increase surface area, along much of its length. Different species of annelids have a wide variety of diets, including active and passive hunters, scavengers, filter feeders, direct deposit feeders that simply ingest the sediments, and blood-suckers. Nervous and Digestive systems 18 Oligochaetes and polychaetes The vascular system and the nervous system are separate from the digestive tract. The vascular system includes a dorsal vessel conveying the blood toward the front of the worm, and a ventral longitudinal vessel that conveys the blood in the opposite direction. The two systems are connected by a vascular sinus and by lateral vessels of various kinds, including in the true earthworms, capillaries on the body wall. The nervous system has a solid, ventral nerve cord from which lateral nerves arise in each segment. Circulatory system hearts 19 Excretory system 20 Excretory system Reproductive system 21 Reproduction Depending upon the species, annelids can reproduce both sexually and asexually. Asexual reproduction Asexual reproduction by fission is a method used by some annelids and allows them to reproduce quickly. The posterior part of the body breaks off and forms a new individual. The position of the break is usually determined by an epidermal growth ,for example, to reproduce by the body breaking into such fragments. 22 Reproduction Earthworms and other oligochaetes, as well as the leeches, are hermaphroditic and mate periodically throughout the year in favored environmental conditions. They mate by copulation. Two worms, which are attracted by each other's secretions, lay their bodies together with their heads pointing in opposite directions. The fluid is transferred from the male pore to the other worm. Different methods of sperm transference have been observed in different genera, and may involve internal spermathecae (sperm storing chambers) or spermatophores that are attached to the outside of the other worm's body. 23 Reproduction Sexual reproduction allows a species to better adapt to its environment. Some annelida species are hermaphroditic, while others have distinct sexes. Most polychaete worms have separate males and females and external fertilization. The earliest larval stage, which is lost in some groups, is a ciliated trochophore, similar to those found in other phyla. The animal then begins to develop its segments, one after another, until it reaches its adult size. 24 Relationships The arthropods and their kin have long been considered the closest relatives of the annelids on account of their common segmented structure. However, a number of differences between the two groups suggest this may be convergent evolution rather than a feature passed on by common descent. The other major phylum that is of definite relation to the annelids is the mollusk, which shares with them the presence of trochophore larvae. Fossil record The annelid fossil record is sparse, but a few definite forms are known as early as the Cambrian. There are some signs they may have been around in the later Precambrian. Because the creatures have soft bodies, fossilization is an especially. 25 Fossil record The annelid fossil record is sparse, but a few definite forms are known as early as the Cambrian. There are some signs they may have been around in the later Precambrian. Because the creatures have soft bodies, fossilization is an especially. 26 27