Phylum Annelids, Biology tutorial

Coelomata - Eucoelomata - Phylum Annelida:

Animals belonging to Phylum Annelida are true coelomates and they are also known as euoelomates. Generally, annelids have elongated body divided externally in number of rings that represent division of interan; paets into the series of segments or metameres.

The Coelom:

Among other things, this fluid - filled space served as the hydrostatic skeleton increasing efficiency of burrowing. Though, in pseudocoelomates, organs lay loose in body cavity. To circumvent this benefit coelom evolved in mesoderm.

The result was that new body cavity, that is the coelom, came to be suspended in coelom by mesodermal layer known as mesenteries also served as the better medium for positioning blood vessles to respective organs. Therefore, growth of coelom has been the major step in evolution of more complex and larger animals.

Metamerism (segmentation):

Coelomates the coelom was not divided in segements, entire body cavity was the single space. Therefore body movements were not precise. Though, with evolution, coelom became divided in a number of chambers by partitions or septa composed of mesodermal lining, peritoneum. This increased efficiency and precision of body movements, as individual segements could be moved more exactly now, with same mechanism that is, hydrostatic or hydraulic pressure. Also, every segment came to have the repetition of several other organ systems such as excretory, circulatory, reproductive and nervous sytems. Each body segment is therefore more or less repetition of other and therefore redundant; animal can survive and function usually even if few segments were lost. This event of divisions of body in series of more or less identical segments each having a section of almost all systems, is called as segmentation or has evolved separately at least twice in animal as the segment or metamere. Metamerism (Annelida - Arhtropoda) and in deuterostomes (vertebrates).

Phylum Annelida comprises of segmented worms. There are approx 15,000 species coming under the phylum. They comprise earth worms, leeches and polychaetes.

Features of Phylum Annelida:

1. Body vermiform, bilaterally symmetrical, with metamerism.

2. Triploblastic, with tisues, organs and organ systems; body wall with outer circular and inner longitudinal layers; epithelium secretes the outer transparent, moist cuticle.

3. Chitinous setae present (except in leeaches)

4. Schizocoelic coelom - coelom appearing as the split or cavity in mesoderm.

5. Blood vascular system closed, often with retory pigments; plasma contains amoebocytes.

6. A presegmental prostomium and post segmental pygidium.

7. Nervous system comprises of supraoesophageal ganglion (cerebral ganglin) circumoesophageal ring and ventral nerve card with segmental ganglia.

8. Sensory sytem comprising of eyes, photoreceptorcells, statocysts, taste buds and tactile orgasm.

9. Excretory system normally comprising of pair of nephridia in every segement.

10. Respiration by skin, gills or parapodia.

Annelid body pattern is basically composed of prostomium and a segmented body followed by pygidium respectively from infont backwards. Prostomium and pygigium are not regarded as segments. Mouth opens on first segements, and anus opens on pygidium. Anterior few segments fuse with prostomium to form head. New segments are added on in front of pygidium, and oldest segements are located anteriorly. Body wall has well developed and strong longitudinal muscles and circular muscles: covering muscular layer is epidermal epithdium that secretes nonchitinous cutilcle. Varying number of chitinous setae present in body wall except in leeches. Coelom originates from embryonic mesodermal cell mass, as split or cavity on either side, and is therefore schizocoetic. Eventually coelom becomes lined by the mesodermal layer, peritoneum. Peritoneal layers on either side meet along median line forming mesenteries that suspend alimentary canal and longitudinal blood vessels.

The excretory organs, nephridia are naturally positioned intersegmentally, one on either side of median line, on septa. Their internal openings into coelom, nephrostomes are positioned in front of septum; the body of nephridium is positioned in segment behind. Typically, each segment has therefore got coelomic chamber, due to hydrostatic pressure. Contraction of the longitudinal muscles results in broadening of the segment; contraction of the circular muscles results in elongation of the segement.

Classification of Phylum Annelida:

Phylum is usually divided in three classes, that is Polychaeta, Oligochaeta and Hirudinea.

1. Class Polychatea: There are generally marine forms with distinctive head, containing eyes and tentacles, segments have Lateral projections of body wall known as parapodia that carry bundles of setae. These animals don't contain clitellium. Sexes are sperate (dioecious). They do not have distinct or permanent sex organs, but their gonads comprise of masses of developing gametes arising as swellings of peritoneum. Eggs generally develop in trochophore larva. Several forms reproduce asexually by budding. Most of the polychaetes are 5 - 10 cm long. They live under rocks and in crevices, or burrow in mud. These animals are generally divided into errant forms on one hand, that are freely moving, pelagic, active burrowers, crawlers and tube worms that leave their tubes for different purposes. In contrast the sedentary forms don't leave their tubes or burrows but may generally expose only their heads outside. These segments also generally carry sensory structures, but their parapodium also bears bundles of chitinous setae in setae in setal sacs. Amphitrite is the example of sedevtary polychaete found intubes built in mud or sand. It feeds on tiny particles of food using long extensible tentacles arising from its head projecting out of its burrow. It has also three pairs of branched gills Sabella is another sedentary polychaete. It extends its crown of tentacles (radioles) from the leathery tube which it secretes, reinforced with sand.

The radioles nerve to every food. Chaetopterus is also sedentary polychaete living in U shaped parchment like tube. It pumps in water by tube by means of three forms. The food particles in stream are entangled by mucus secreted by wing like notopodia of 12th segment.

2. Class Oligoachaeta Mostly living in soil, or in fresh water, conspicuous body segmentation but no distinct head; body variable number of segements. Number of setae in every metamere fewer. Coelom spacious and separated by intersegmental septa; hermaphrodites (monoecious). Reproductive system more complicated, compact ovaries and testes but fewer in number. No larva, development direct. Earthworms are most familiar animals in group, burrowing in soil, enriching it and producing worm casts. These are quite larger, generally 12- 30cm long and have 150 - 250 segments or more. They come out of burrows at night. Other group of oligochaetes is aquatic, mostly fresh water, being very small. Locomotion in earthworms is by peristaltic movement, setae being utilized for anchoring body. Body plan of earthworms is extraordinarily constant. The cross section of body shows the thin but water - proof cuticle over epidermal layer.

They feed on decayed organic matter. Muscular pharynx sucks in moistened food. Calciferous glands along oesophagus control calcium ions of blood by secreting excess calcium into gut.

3. Class Hirudinea. These are leeches. Number of body segments of the animals is fixed; generally there are 34 segments; in some groups there may be just 31 or even 17. Segments have several small annuli. Anterior and posterior sucker present, as also clitellum. No parapodia or setae. Coelom filled having connective tissue and muscles. Hermaphrodites; direct development, may be terrestrial, fresh water or even marine. Leeches differ in size from 2- 6cm in length, and are flattered dorsoventrally. Clitellum, thought present, seems only during breeding season. Their gut is very specialized for storage of blood. Though they contain only 34 segments, as segments are marked by transverse grooves they seem to have more rings (annuli). With regard to coelom, septa have disappeared, and coelom is filled with a connective tissue (botryoidal tissue). Remaining space known as lacunae are filled with coelomic fluid. Locomotion is by looping movements, through two suckers, or by undiluating, movements in water. Majority of them feed on blood and are bloodsuckers of either warm-blooded or cold blooded vertebrates, and contain jaws for cutting tissues. Major excretory organs are nephridia. Brains comprise of ring of gangalia around pharynx and double ventral nerve and with number of gangalia. They are hermaphrodites, but perform cross fertilization. Cocoon secreted by clitellum receives eggs and sperms. It is deposited in mud. Examples: Hirudo medicinalis, medicinal leech; Glossiphonia.

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