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Phylum Annelida: summary of characteristics. Name from Latin a nnulus meaning a ring. Vermiform. Possess tissues and organs. Muscular gut with mouth and anus. Body divided into segments. Outer epithelium with clumps of bristles (except in forms with suckers).
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Phylum Annelida: summary of characteristics • Name from Latin annulus meaning a ring. • Vermiform. Possess tissues and organs. • Muscular gut with mouth and anus. • Body divided into segments. • Outer epithelium with clumps of bristles (except in forms with suckers). • Body wall muscular with both circular and longitudinal muscles. • Closed circulatory system. • Nervous system with supraoesophageal ganglion, circum-oesophageal ring and ventral nerve cord. • Nephridia responsible for most excretion
Phylum Annelida • The annelids (L. annelus: a little ring) are the segmented worms. • Annelids are coelomate, protostomes and the body is metameric being composed of serially repeated segments or metameres. • Each segment is separate from the next segments being divided by partitions or septa.
Segmentation • Within each segment are components of most organ systems such as the circulatory, nervous and excretory systems. • Thus, there is a degree of redundancy in annelids so that if a segment is damaged it need not be fatal.
Segmentation • The evolution of segmentation is the great evolutionary innovation of the annelids. • Segmentation allows annelids to make more precise body movements than organisms that have a hydrostatic skeleton, but lack segmentation e.g. the nematodes.
Segmentation • Because the coelom is divided by septa the force of muscle contraction in a segment is not transmitted throughout the body, but instead is confined to the single segment. • Thus, one segment may elongate while the adjacent one contracts and this allows the animal to make fine, controlled movements.
Movement • With the exception of the leeches, the coelom is filled with fluid and acts as a hydrostatic skeleton. • Annelids possess circular and longitudinal muscles and this enables individual segment to be elongated or contracted. • Crawling is achieved by alternating waves of contraction by circular and longitudinal muscles passing down the body (peristalsis).
Movement • Because they have fine control of movement annelids have evolved a relatively sophisticated nervous system. • Most annelids are burrowing forms and as an adaptation to this lifestyle bear short chitinous bristles called setae on each segment. The setae enable the annelid to gain traction against the side of the burrow.
Movement • In other annelids longer hair-like setae assist the animal in swimming. • For the annelids that live in burrows or in tubes the setae help to prevent the animal from being pulled out.
Annelids • Annelids occur worldwide being found in the sea, freshwater, and in the soil. • They feed on organic matter in the mud or soil, by filtering suspended particles from the water, act as predators, or suck blood.
Annelids • The typical annelid body has a two part head made up of a prostomium and a peristomium, a series of segments, and a terminal pygidium which contains the anus. • Neither the head nor the pygidium are considered true segments. In growth, new segments form anterior to the pygidium. If an annelid is cut in two the posterior segments can be regrown.
Annelid Classification • There are approximately 12,000-15,000 species of annelids divided into 4 classes: • Polychaeta: polychaete worms • Oligochaeta: earthworms • Hirundinea: leeches • Siboglinidae: pogonophorans
Class Polychaeta • The polychaetes are the largest of the annelid classes and include more than 10,000 described species, most of which are marine. Morphologically very diverse. • The name “poly” “chaete” refers to the numerous chaetae or bristles they possess.
Polychaetes • Polychaetes have a well differentiated head that has sense organs including eyes and cirri (short tentacles), jaws (in predatory forms), or a fan for filter feeding. • Most segments bear parapodia, which are lobed structures used in swimming, crawling, or for anchorage in tubes. Parapodia also serve as gills.
Class Polychaeta • Polychaetes follow one of two basic lifestyles being either sedentary/sediment burrowing (“sedentary”) or active hunting (“errant”) species. • Sedentary polychaetes usually exhibit variation in the structure of segments. All are filter-feeders or deposit feeders. • Sedentary polychaetes burrow in mud and soil or build their own tubes from which they filter feed. • Tubes may be made from calcium carbonate, a secreted paper-like material, or sand grains.
Fanworms • Most of the sedentary polychaetes, which inhabit burrows or build tubes, are filter feeders and consume plankton or detritus. • Forms such as fanworms extend long, modified feathery crowns of stiff prostomial tentacles to feed. Ciliary action draws in food, which is trapped in mucus and delivered down grooves to the mouth.
Burrowing polychaetes • A number of families of sedentary polychaetes burrow in soft sediments either swallowing sediment or scraping it of bacteria, algae, fungi and other live material. • Many functionally resemble oligochaetes and have reduced parapodia, lack prominent sense organs and have well developed circular muscles and septa. • Some have soft prehensile tentacles they use collect food particles.
http://www.nw1design.com/clients/afen/images/pics/Polychaete.jpghttp://www.nw1design.com/clients/afen/images/pics/Polychaete.jpg
Burrowing Polychaetes • Burowing polychaetes such as lugworms are very common on estuaries. • They make burrows in the sand and consume large quantities of sand. After they’ve extracted the digestible material the remaining material is defecated and forms a characteristic pile outside the burrow.
Lugworm (two images above) from http://marinebio.org/species.asp?id=57 Above right Lugworm casts. http://upload.wikimedia.org/wikipedia/commons/7/7b/Lugworm_cast.jpg
Burrowing Polychaetes • Lugworms are an important source of food for wading birds. http://cache2.asset-cache.net/xc/88392324.jpg?v=1&c=IWSAsset&k=2&d= EDF6F2F4F969CEBD9BAF6D58632300DBDC286D88A6C0AC12DB7DFB94F05A5839
Predatory polychaetes • Predatory forms of polychaetes such as Nereis have a muscular pharynx equipped with jaws that can be quickly everted to grab prey.
http://weblog.greenpeace.org/ defendingourmediterranean/images/ 180polychaete_worm_hermodice_carunculata __whole_worm_for_hibsy.jpg Bobbit worm a predatory polychaete. http://www.tonywublog.com/20090319/fright-night.html
Predatory polychaetes • Predatory polychaetes typically can crawl rapidly using their parapodia. • They are active hunters that can sqeeze through small spaces (e.g. in coral, crevices, etc.) is search of prey. • They consume any other invertebrates that they can catch and dismember.
Class Oligochaeta • There are over 3000 species of oligochaetes, the most familiar of which are the earthworms. • Lumbricusterrestris, the common earthworm, grows from 4-12 inches, but tropical forms may reach 12 feet in length.
Class Oligochaeta • Earthworms burrow in rich, damp soil and leave their burrows at night to eat vegetation and to breed. • Earthworms play a significant role in soil fertility by aerating the soil with their burrows, adding vegetable material, and mixing subsoil and topsoil.
http://www.cheshirewildlifetrust.co.uk/IMAGES/watch_earthworm.jpghttp://www.cheshirewildlifetrust.co.uk/IMAGES/watch_earthworm.jpg
Class Oligochaeta • Darwin studied earthworms and published a book on their effects on soil. • He estimated that an earthworm eats its own weight in soil daily and that in an acre of land 10-18 tons of dry soil passed through their guts annually. • Earthworms consume dead organic material and partially digest it, the waste passing out of them containing nutrients valuable to plants and supplemented with nitrogenous wastes from the worm.
Class Oligochaeta • In addition to the earthworms there are many freshwater species, most of which burrow in silt and mud or creep along the bottom, although some live among submerged vegetation. • Freshwater forms usually are smaller than terrestrial and have more conspicuous setae. • Most respire through their skins, but some have gills. Most are algae or detritus feeders.
11.16 Freshwater oligochaetes
Class Oligochaeta • Oligochaetes, like all annelids, have a double circulatory system as both the coelomic fluid and circulatory system are used to carry food, wastes and gases. • The blood system is closed, with the dorsal blood vessel being the main pumping organ.
Class Oligochaeta • The excretory organs are called nephridia and there is a pair in each segment, each of which occupies parts of two successive segments. • A ciliated funnel (the nephrostome) opens just anterior of an intersegmental septum and from this a tubule leads into the posterior segment and forms a series of loops that are closely surrounded by blood vessels.
Excretory organs • The tubule eventually opens to the outside via an aperture called a nephridiopore. • The system works by cilia drawing coelomic fluid into the nephrostome and selective reabsorbtion of salts and water occurs in the loops leaving only a dilute urine to be excreted to the outside.
Reproduction in earthworms • Earthworms are hermaphroditic and mate by aligning their ventral surfaces together. • Each worm’s clitellum (thickened section of some midbody segments) secretes mucus, which holds the two worms together. • Sperm is exchanged and stored in a seminal receptacle.
Reproduction in earthworms • After sperm has been exchanged the worms separate and each secretes a cocoon around its clitelleum. The cocoon slides along the body and picks up eggs and sperm. • Fertilization occurs within the cocoon as does later embryonic development. • As the cocoon slides off the worm its ends seal. Young worms emerge several weeks later.
Class Hirudinea • There are more than 500 species of leeches, most of which are freshwater inhabitants. • Leeches have anterior and posterior suckers which they use in locomotion. With the exception of one group, leeches lack septae and their coelom is largely filled with connective tissue and muscle.
Class Hirudinea • Many leeches are carnivorous, but leeches are best known as blood-sucking ectoparasites. • The leech penetrates its host using its jaws or proboscis and sucks blood with its powerful pharynx. • To ensure blood continues to flow the leech secretes a powerful anticoagulant (hirudin) in its saliva.
World’s largest leech Haementeria ghilianii 11.17
