Phylum Annelida: summary of characteristics
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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). May be covered with a cuticle.
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 pseudocoelomate 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.

Figure 17.03a
11.1

Figure 17.03c

Annelid Classification
 There are approximately 12,000-15,000 species of annelids divided into 4 classes:
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Polychaeta: polychaete worms
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Oligochaeta: earthworms
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Hirundinea: leeches
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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.

Figure 17.03e

Figure 17.03c

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.

Figure 17.02b
11.3B

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.

Figure 17.02a
11.3A

Figure 17.10
11.7

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.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.

Figure 17.03a
11.1

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.

Figure 17.09
11.6

Class Oligochaeta
 There are over 3000 species of oligochaetes, the most familiar of which are the earthworms.
 Lumbricus terrestris, 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.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.

Figure 17.18
Freshwater oligochaetes
11.16

Class Oligochaeta
 Oloigochaetes, 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.

http://z.hubpages.com/u/94165_f520.jpg

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.

Figure 17.14
11.14

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.

Figure 17.17
11.15

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.

Figure 17.20
11.18

Figure 17.19
World’s largest leech
Haementeria ghilianii
11.17

Class Hirudinea
 For hundreds of years leeches were used for blood letting, in the belief that too much blood caused a variety of medical conditions
 After being discarded as a medical tool leeches are again being used by surgeons.

Class Hirudinea
 In reattachments of severed digits and in the case of skin grafts, because the blood vessels are damaged, pooling of blood often threatens to kill the attached tissue.
 Leeches, however, can remove the pooling blood safely allowing time for veins to develop.

Figure 17.21
11.19
Medicinal leech feeding

Class Siboglinidae
(pogonophorans)
 The pogonophorans (or beardworms) were formerly considered to be a phylum, but now are considered to be derived from the polychaetes.
 These were first discovered during deep sea dredging in 1900 off Indonesia, but since then about 80 species have been identified in seas worldwide.

Class Siboglinidae
(pogonophorans)
 While similar to tube dwelling polychaetes, the first pogonophorans were considered to be a separate group because they lack a complete gut and appeared not to be segmented.

Class Siboglinidae
(pogonophorans)
 The lack of segmentation proved to be illusory.
Pogonophorans live buried in the mud and their lower ends were broken off when collected during dredging.
 In 1964 complete pogonophorans were dredged up and it was discovered that the posterior end of pogonophorans (called the opisthosoma) is segmented and bears setae.

Class Siboglinidae
(pogonophorans)
 Most siboglinids live in the mud and silt of the seafloor usually at depths > 200m.
 The body is divided into a short forepart, which bears tentacles, a long, slender trunk, and the small segmented opisthosoma. The body is covered with a cuticle and has setae on the trunk and opisthosoma.

Siboglinum fiordicum
11.11
Opisthosoma

Class Siboglinidae
(pogonophorans)
 Because siboglinids have no mouth or complete gut it’s unclear how they obtain nutrition.
 They absorb some nutrients in the water through their tentacles, but most energy apparently is derived from a mutualistic association with chemoautotrophic bacteria.
 The bacteria oxidize hydrogen sulfide to produce energy and live in an expanded section of the midgut called a trophosome. There is no foregut or hindgut.

Phylum Echiura
 The Echiura (from Greek -echis : a viper and ura : a tail) are worms that are closely related to the Annelids.
 Like annelids thay have a trochophore larva, but differ from the annelids in being unsegmented.
 They are widely distributed in shallow marine benthic habitats.
 Possess a characteristic extensible proboscis (used in feeding on detritus) and a set of small hooks or spines on the tail.
 Echiurans live in permanent burrows in soft sediments. Most are unselective detritus feeders

Echiura: http://www.usp.br/cbm//images/rsgallery/display/echiura02.JPG.jpg

Phylum Echiura
 Urechis caupo the “innkeeper worm” is a common inhabitant of mudflats along the coast of California.
 It builds and lives permanently in a U-shaped burrow and it uses a mucus net secreted by its proboscis to trap plankton in water it draws through its burrow.
 Urechis is called the "innkeeper worm" because many marine organisms, such as small crustaceans, polychaete worms and fish, live commensally inside its burrow.

Urechis caupo the Innkeeper worm http://www.ryanphotographic.com/images/JPEGS/Echiuran.jpg

Phylum Sipuncula
 The Phylum Sipuncula (from Latin: meaning
“little pipe”) consists of approximately 250 species of benthic, marine worms, most from 15-
30 cm in length.
 Sometimes referred to as the “peanut worms” most burrow in sand or silt or occupy crevices or empty mollusc shells or worm tubes.

http://www.glaucus.org.uk/Sipunculus-nudus-RL.jpg

Phylum Sipuncula
 The body is unsegmented and divided into an anterior introvert and a posterior trunk.
 Like the Echiurans the sipunculids are generally non-selective deposit feeders and they use the tentacles surrounding the tip of the introvert to collect food.
 They produce a trochophore larva similar in structure to that of the annelids.

http://www.brookscole.com/chemistry_d/templates/student_resources/0030244269_cam pbell/images/hottopics/Sipunculida.gif