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| Organization of the body into a series of ringlike segments.
Body is divided internally as well.
Segmental arrangement of body parts in an animal is called metamerism
For example, the compartmentalization of the body has resulted in each segment having its own excretory, nervous, and circulatory structures.
Two related functions are probably the primary adaptive features of metamerism: flexible support and efficient locomotion.
Enlargement of each cavity forms a double membrane septurm on the anterior and posterior margin of each coelomic space and dorsal and ventral mesenteries associated with the digestive tract.
The segmental arrangement of coelomic spaces and muscles creates hydrostatic compartments and makes possible a variety of advantageous locomotor and supportive functions not possible in nonmetameric animals that utilize a hydrostatic skeleton.
Each segment can be controlled independently of distant segments, and muscles can act as antagonistic pairs within a segment.
A second advantage of metamerism is that it lessens the impact of injury.
If one or a few segments are injured, adjacent segments, set off from injured segments by septa, may be able to maintain nearly normal functions.
A third advantage of metamerism is that it permits the modification of certain regions of the body for specialized functions.
The specialization of body regions in a metameric animal is called tagmatization.
Class Oligochaeta
3,000 species
Aquatic species live in shallow water, where they burrow in mud and debris.
Terrestrial species are found in soils with high organic content, and these species rarely leave their burrows.
Soil-conditioning habits of earthworms are well known
Lumbricus terrestris was introduced to the US from northern Europe.
Oligochaetes have setae, but fewer than are found in polychates.
Oligochaetes lack parapodia, because parapodia and long setae would interfere with their burrowing lifestyles.
The prostomium consists of a small lobe or cone in front of the mouth and lacks sensory appendages.
Clitellum that is used for mucous secretion during copulation and cocoon formation.
The body is covered by a nonliving, secreted cuticle.
Oligochaete locomotion involves the antagonism of circular and longitudinal muscles in groups of segments.
Neurally controlled waves of contraction move from rear to front.
Segments bulge and setae protrude when longitudinal muscles are contracted, providing points of contact with the burrow wall.
In front of each region of longitudinal muscle contraction, circular muscles contract, causing the setae to retract, and the segments to elongate and push forward.
During burrowing earthworms swallow considerable quantities of soil.
Oligochaetes are scavengers and feed primarily on fallen and decaying vegetation, which they drag into their burrows at night.
The mouth leads to a muscular pharynx.
In the earthworm, pharyngeal muscles attach to the body wall.
The pharynx acts as a pump for ingesting food.
The esophagus is narrow and tubular and frequently is expanded to form a stomach, crop, o gizzard;
A crop is a thin walled storage structure, and a gizzard is a muscular, cuticle lined grinding structure.
Calciferous glands are evagination of the esophageal wall that rid the body of excess calcium absorbed form food.
the intestine is a straight tube and is the principle site of digestion sand absorption.
The surface area of the intestine is increased substantially by a dorsal fold of the lumenal epithelium called the typhlosole
The intestine ends at the anus.
Respiratory gases of most annelids simply diffuse across the body wall, and parapodia increase the surface area for these exchanges.
The circulatory system of annelids is a closed system.
Oxygen is usually carried in combination with molecules called respiratory pigments, which are usually dissolved in the plasma rather than contained in blood cells.
Contractile elements of annelid circulatory systems consists of a dorsal aorta that lies just above the digestive tract and propels blood from rear to front, and a ventral aorta that lies ventral to the digestive tract and propels blood from front to rear.
The ventral nerve cord and all ganglia of oligochaetes have undergone a high degree of fusion.
An escape response results from the stimulation of either the anterior of the posterior end of a worm.
Oligochaetes lack well developed eyes
Other oligochaetes have simple pigmentcup ocelli, and all have a dermal light sense that arises from photoreceptor cells scattered over the dorsal and lateral surfaces of the body.
Scattered photoreceptor cells mediate a negative phototaxis in strong light and a positive phototaxis in weak light
Oligochaetes are sensitive to a wide variety of chemical and mechanical stimuli.
Receptors for these stimuli are scattered over the surface of the body, especially around the prostomium.
Oligochaetes use metanephridia for excretion and for ion and water regulation.
As with polychaetes, funnels of metanephridia are associated with the segment just anterior to the segment containing the tubule and the nephridiopore.
ammonia and urea.
Oligochaetes excrete copious amounts of very dilute urine, although they retain vital ions.
Chloragogen tissue acts similarly to the vertebrate liver.
It is a site of amino acid metabolism.
Chloragogen tissues also convert excess carbohydrates into energy storage molecules of glycogen and fat.
All oligochaetes are monoecious, and usual sperm exchange occurs during copulation.
During copulation of Lumbricus, two worms line up facing in opposite directions, with the ventral surfaces of their anterior ends in contact with each other.
THis orientation lines up the clitellum of one worm with the genital segments of the other worm.
Worms are held in place by a mucous sheath, secreted by the clitellum., that envelopes the anterior halves of both worm.
Following copulation, the clitellum forms a cocoon for the deposition of eggs and sperm.
The cocoon consists of mucoid and chitinous materials that encircle the clitellum.
A food reserve, albumen, is secreted into the cocoon by the clitellum, and the worm begins to back out of the cocoon.
Eggs are deposited in the cocoon as the cocoon passes the openings to the oviducts, and sperm are released as the cocoon passes the openings to the seminal receptacles.
Fertilization occurs in the cocoon and as the worm continues backing out the ends of the cocoon are sealed and the cocoon is deposits in moist soil
Hatching occurs in 1 to a few weeks.
Class Polychaeta
mostly marine, and are usually between 5 and 10 cm long
5,300 species and the largest of the annelid classes
A bucket of intertidal sand normal yields vast numbers and an amazing variety of these burrowing annelids.
In addition to metamerism, the most distinctive feature of the polychaetes is the presence of lateral extensions called parapodia.
Parapodia are supported by chitinous rods, and numerous setae project from the parapodia, giving them their class name.
Setae are bristles that are secreted form invagination s of the distal ends of parapodia.
They aid locomotion by digging into the substrate and are also used to hold a worm in its burrow or tube.
The prostomium of a polychaete is a lobe that projects dorsally and anteriorly to the mouth and contains numerous sensory structures, including eyes, antennae, palps, and ciliated pits or grooves, called nuchal organs.
The first body segment, the peristomium surrounds the mouth and bears sensory tentacles or cirri.
Class Hirudinea
500 species
Leeches prey on small invertebrates or feed on body fluids of vertebrates.
Leeches lack parapodia and head appendages. They are dorsoventrally flattened and taper anteriorly
Anterior and posterior segments are usually modified into suckers.
The musculature of leeches is more complex than that of other annelids.
A layer of oblique muscles is present between the circular and longitudinal muscle layers.
These modifications have resulted in altered patterns of locomotion.
Rater than being able to utilize independent coelomic compartments, the leech has a single hydrostatic cavity and uses it in a looping type of locomotion.
Many leeches feed on body fluids or the entire bodies of other invertebrates.
Some fed on blood of vertebrates, however, the association between a leech and its host is relatively brief.
Therefore, describing leeches as predatory is probably more accurate.
While feeding, a leech attaches to its prey by the anterior sucker and either extends its proboscis into the prey or uses its jaws to slice through host tissues.
Salivary glands secrete an anticoagulant called hirudin that prevents blood from clotting.
Behind the mouth is a muscular pharynx that pumps body fluids of the prey into the leech.
The esophagus follows the pharynx and leads to a large stomach with lateral ceca.
After engorgement, a leech can tolerate periods of fasting that may last for months.
The digestive tract ends in a short intestine and anus.
Gas exchange occurs across the body wall.
the nervous system of leeches is similar to that of other annelids.
All leeches reproduce sexually and are monoecious,.
None ar capable of asexual reproduction or regeneration.
Sperm transfer and egg deposition usually occur in the same manner as described for oligochaetes.
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