Annelids are usually triploblastic and coelomate. They are bilaterally symmetrical and metamerically segmented. There is lip-like extension of first segment known as prostonium. There is outer covering of cuticle and hair-like, chitineous chaetae set segmentally in all except the leeches.
The Coelomate Body Plan:
Aceolomate means having no ceolom. Platyhelminthes, third layer, mesoderm that is also middle layer completely filled space between ectoderm and endoderm. This way the solid middle layer is formed. In annelids, there is internal space known as coelom. This space is formed from splitting of mesoderm in 2 layers -somatic mesoderm to outer part and splanchnic mesoderm to inside, together with and attached to outer wall, somatic mesoderm from body wall. While together with and attached to endoderm, splanchnic mesoderm form muscular wall of gut. Hence it can be stated as coelom divides body wall from gut wall. Most of mesoderm that lives coelom develops into muscles. The one of the body wall permits movement of complete animal. That of gut causes the wave-like contraction of muscles of gut wall that forces food to move on along it. Transportation of every material between gut and body wall take place through vascular system (which in animal is blood). A vascular system in plants, were made up of phloem and xylem cells. In animals, hole (cavity) in gut is endoderm.
Surface lining of coelom is peritoneum. These parts of which join wall of gut to that of body across coelom are known as mesenteries. Organs like reproductive or excretory organs that project into coelom are enclosed by peritoneum. To understand the description of coelom, assume a thick pipe. The space inside pipe is endoderm. Thick part which is solid substance of thick pipe is the mesoderm that in platyhelminthes stays solid and makes them acoelomate. In annelids, solid substance is cut open to carry the cavity. This hole is coelom. The result of this is that there are now 2 cavities and 2 solid parts (layers) in pipe. Holes and solid alternate each other. Innermost hole is original cavity in pipe. New one, middle between 2 solid layers is coelom. Mesentery though joins 2 solid parts now formed by split together. It is like the bridge (or a canal). Solid part joined to ectoderm is known as somatic mesoderm. That attached to endoderm is known as splanchnic mesoderm. Original cavity in pipe is gut. From somatic mesoderm, few organs protrude into coelom. There are tubes from these organs to exterior.
Biological significance of Coelom:
Due to coelom now divided gut wall from body wall, movements affecting 2 parts are also divided. Organism can have their movements independent of external movements. In annelids, coelom serves as the hydrostatic skeleton (a fluid skeleton). Skeletons serve 3 purposes, which are protection, locomotion and support. Due to this incompressible fluid class oligochaeta, earthworms can modify their shape without varying its volume. Muscles apply pressure on body fluid to cause longer thinner and broader thicker portions of body segment alternately. Fluid assist disperse external forces quickly therefore giving protection to animal. With bigger size and distances from different parts of body, resulting from presence of the coelom, is also requirement for transport system. This in turn signifies greater complexity of body that requires to be coordinated by the nervous system.
This is the other evolutional progress in coelomates. Metameric segmentation is transverse division of body into related parts. This division causes development of the long body with similar segments. Division affects both mesoderm and ectoplasm. Externally this segment seems like constrictions of body surface. Internally segments are divided from each other by septa extending across coelom. Each segment has its own blood vessels, block of muscles, nerve cells and in some groups reproductive organs. Segments are not completely independent in annelids the nervous and the excretory system run-down all the segments. The emergence of coelom and segmentation set down basic plan for evolutionary development that come in various ways in segments are group of segments. Specialization and division of labour can also occur. For instance in cephalization numerous segments fuse together to form head. In Arthropods it is loss of cells.
Size, Surface, Area: Volume Ratio
Flat worms and other smaller animals having bacteria and viruses have extremely large surface area volume ratio. They can depend on diffusion for the lot of their life process - nutrition, gaseous exchange and excretion.
Though as sizes increase, diffusion can no longer suffice, as surface are - volume ratio drops rapidly. Coelomates should have special organs for gaseous exchange and transportation. Common characters of annelids and evolutionary advances have over platyhelminthes.
Classification of Annelids:
Annelids are classified into three classes - the Polychaeta, Oligochaeta and Hirundinea.
The Oligochaeta (Earthworm):
The earthworm is an oligochaeta. The earthworm is extremely significant to man due to effects of its burrowing activities on soil. Earthworm has not completely overcome all problems of living on land. To prevent aridity, it burrows into damp soil where it can live for along only comes out at night to feed and reproduce. It feeds on remains of decomposing organic material by swallowing soil. Majority of what comes out of worm as faeces is indeed soil with which it forms its casts. This is very fertile.
Structure and Adaptation:
The earthworm is lengthened, cylindral and between 12 - 18 cm long. It is pointed interiorly and compressed posteriorly. It has mouth at front end and anus at tail. It has no projecting structures on body which might slow down its burrowing into soil. There is small rounded prostonium hanging over mouth. Every segment has 4 pairs of chaetae, 2 ventrally and 2 ventrolaterally except first and last. The chaetae protrude from sacs in body wall that can be protruded and withdrawn by specialized locomotory muscles. There are longer chaetae that are utilized during copulation. Clitellum (the saddle formed by gland cells covered by epidermis) is also discovered. It is helpful in copulation and formation of cocoon.
It keeps itself moist with secretion of coelomic fluid from dorsal pores. Mucus also from mucous gland also help keep worms cuticle moist, prevent aridity, ease gaseous exchange and serve as lubricant as worm moves through soil. There are also networks of looped blood capillaries in epidermis through which gaseous exchanges occur by diffusion over epidermis kept moist and coelonic secretions. There is pair of excretory and osmoregulatory tubes (neptiridia) situated in every segment except first three and last one. They open on to surface of animal through pores. Earthworm is hermaphrodite. This signifies that it has both male and female sex organs in same animal. .
Polychaetes are generally marine but few are freshwater or brackish water species. Generally originate from intertidal zone to depths of over 5,000 m. The polychaetes are so named due to several setae or chaetae they allow. They have approx 8,000 species. Body segment contain pair of locomotory organs known as parapodia which bear several bristles known as chetae, due to which polychaetes are at times stated to as bristle worms. Body size differs from 1 mm to approx one metre. Polychaetes vary from other annelids in containing well distinguished head with sense organs, like eyes, antennae and sensory palps and there is no clitellum on body.
The leech is the example of class Hirundea is even more dissimilar. Whereas oligochaetes have no parapodia, they have some chaetae. Hirundea have neither. As they are ectoparasites, leech are only prepared with suckers to join themselves to the prey. This is diagnostic feature of the class.
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