How many annelids

Metamerism increases the efficiency of body movement by allowing the effect of muscle contraction to be extremely localized, and it makes possible the development of greater complexity in general body organization. Besides being segmented, the body wall of annelids is characterized by being made up of both circular and longitudinal muscle fibers surrounded by a moist, acellular cuticle that is secreted by an epidermal epithelium.

All annelids except leeches also have chitonous hair-like structures, called setae, projecting from their cuticle. Sometimes the setae are located on paddle-like appendages called parapodia. Annelids are schizocoelous and with a large and well-developed true coelom i. Except in leeches, the coelom is partially subdivided by septa. Hydrostatic pressure is maintained across segments and helps maintain body rigidity, allowing muscle contractions to bend the body without collapsing it.

The internal organs of annelids are well developed. They include a closed, segmentally-arranged circulatory system. The digestive system is a complete tube with mouth and anus. Indeed, the gills of tube-dwellers and burrowers usually cluster around whichever end has the stronger water flow. The circulatory system in an earthworm is a closed system and basically is a loop. Instead of a true heart, earthworms have five aortic arches that pump blood through the dorsal blood vessel; it then returns through the ventral blood vessel.

Annelids with blood vessels remove soluble waste products with metanephridia, while those without use protonephridia. Both of these nitrogenous waste systems use a two-stage filtration process, in which fluid and waste products are first extracted and then filtered again to reabsorb any reusable materials while dumping toxic and spent materials as urine. Earthworms have no central brain structure, but rather possess two cerebral ganglia that are situated in the anterior portion of the body.

The ganglia are attached to a ventral nerve cord that runs the length of the animal. Earthworms are able to sense chemicals, moisture, light, temperature, touch, vibrations, and tastes. With regard to a reproductive system, most mature clitellates such as earthworms and leeches are fully hermaphroditic animals, and thus they possess both male and female reproductive organs on the same organism.

The exception is that in a few leech species, younger adults function as males and then, at maturation, become females. Some oligochaetes, such as Dero Aulophorus furcatus , appear to reproduce entirely asexually, while some others reproduce asexually in the summer and sexually in fall. Polychaetes can reproduce asexually by dividing into two or more pieces or by budding off a new individual while the parent remains a complete organism.

All clitellates have well-developed gonads, and all copulate. However, they cannot fertilize themselves and must reproduce with another earthworm. To accomplish this, two earthworms must line up opposite of each other and use their setae to grip one another before engaging in copulation. Earthworms are able to store the sperm of the partner.

The clitellum secretes mucous that becomes a cocoon in which the eggs remain for two to three weeks before hatching. The eggs of leeches are fertilized in the ovaries and then transferred to the cocoon.

In addition, the cocoon also either produces yolk when the eggs are fertilized or nutrients while they are developing. Earthworms and leeches hatch as miniature adults rather than producing a larval stage.

Earthworms play an integral part in the ecology of the terrestrial environment. They generally burrow into the soil , but some live in moist leaf litter.

They loosen the soil so that oxygen and water can penetrate it, and both surface and burrowing worms help to produce soil by mixing organic and mineral matter by accelerating the decomposition of organic matter and making it more quickly available to other organisms, and by concentrating minerals and converting them to forms that plants can use more easily.

Earthworms function as valuable prey for a variety of vertebrates, including amphibians , reptiles , birds , and mammals. The burrowing of marine polychaetes encourages the development of ecosystems by enabling water and oxygen to penetrate the ocean floor.

Annelids have an important interplay with humans. By helping farmers and gardeners, they are best known for aerating the soil. In addition, earthworms have been used as fish bait since ancient times. Other annelids such as leeches have also been important to humans through the centuries.

Leeches may attach to humans and animals and suck blood, in some cases transmitting flagellates that can be pathogenic.

Since ancient times, there are accounts of using medicinal leeches Hirudo medicinalis in blood-letting. Interestingly, leeches are still being used in medicine in the twenty-first century, such as assisting in microsurgery, as their saliva has provided anti-inflammatory compounds and several important anticoagulants, one of which also prevents tumors from spreading. Because of the ease in which anglers spread invasive terrestrial earthworms, numbers of North American native earthworms have been drastically reduced over the years.

In the twenty-first century, throughout portions of North America, introduced earthworms are more common. For example, in the glaciated portions of North America, almost all native earthworms are thought to have been killed by the glaciers, and the worms currently found in those areas are all introduced from other areas, primarily from Europe, and, more recently, from Asia. Northern hardwood forests have especially been harmed by invasive worms through the loss of leaf duff, loss of soil fertility, changes in soil chemistry, and loss of ecological diversity.

The introduced earthworm Amynthas agrestis is especially destructive, and one state Wisconsin has listed it as a prohibited species. It is considered to be possibly extirpated. Diplocardia sylvicola was used in a study of residue and burning interaction with tillage to influence earthworm densities in the Lower Mississippi River Valley Delta region of eastern Arkansas.

Except for numerous recent studies on leeches that included twenty-two species from five families reported from the northern part of the state, Arkansas has been scarcely studied for other annelids. Two early publications in the s listed twenty-one species within nine genera of earthworms from Arkansas. Of these, ten species were regarded as native to the United States, eight were introduced European forms, and three were Asiatic in origin.

Representatives of this lineage are characterized by having internal supporting chaetae, or aciculae, in the parapodia. It includes major groups such as Phyllodocida and Eunicida, which tend to be mobile forms with well developed eyes and parapodia for rapid locomotion. Canalipalpata, a group with more than named species, is distinguished by having long grooved palp structures that are used for feeding. Canalipalpata is divided into Sabellida, Spionida and Terebellida.

The monophyly of Annelida is not well supported and only two morphological features are worthy of discussion; segmentation and chaetae. Nuchal organs represent another possible apomorphy and are discussed in the section on sensory structures see plesiomorphies and other features. Annelids have three body regions Fig. The majority of the body is comprised of repeated units called segments. Each segment is, in principle, limited by septa dividing it from neighbouring segments, and has a fluid-filled cavity within referred to as a coelom.

Structures such as the excretory, locomotory and respiratory organs are generally repeated in each segment. Segments are formed sequentially in annelids and are established during development from growth zones located at the posterior end of the body; so the youngest segment in the body of an annelid is always the most posterior.

The only parts of the annelid body that are not segmental are the head and a terminal post-segmental region called the pygidium. The head is comprised of two units, the prostomium and the peristomium. The postsegmental pygidium includes the zone from which new segments are proliferated during growth. The proposed homology of segmentation seen in annelids with that seen in Arthropoda has been used to unite the two as Articulata, a grouping that dates back to Cuvier The homology of this segmentation has been questioned recently, with arthropods now viewed by many as closer to taxa such as Nematoda Aguinaldo et al.

This suggests that the form of segmentation seen in annelids may in fact represent an apomorphy. With regards to the supposedly unsegmented Echiura, their reinstatement within Annelida see McHugh suggests that their apparently unsegmented body in fact represents a series of fused segments see Hessling and Westheide Figure 2.

Ophryotrocha Dorvilleidae. Sandgerdi Iceland. A distinctive feature of annelids are structures called chaetae Fig. Chaetae also called setae are bundles of chitinous, thin-walled cylinders held together by sclerotinized protein.

They are produced by a microvillar border of certain invaginated epidermal cells and so can be defined as cuticular structures that develop within epidermal follicles. Chaetae show a huge amount of variation, from long thin filaments capillary chaetae to stout multi-pronged hooks Fig. Apart from annelids, chaetae are found in Echiura and Brachiopoda.

There is now good evidence Hessling and Westheide, ; McHugh, that the former group falls within Annelida. There is a distinct possibility therefore that chaetae represent an apomorphy for Annelida. Figure 3. Proscoloplos Orbiniidae. Bondi, Australia. SEM and Light micrographs. The most recent comprehensive systematization of polychaetes, that proposed by Rouse and Fauchald from their cladistic parsimony analyses, has been used here Fig. Allowing for the likely errors in the placement of many taxa, and the fact that there were conflicting results included in the original analyses, the most fundamental problem inherent in the systematization used here may be that of the placement of the root for any tree of Annelida.

Their trees also excludes Echiura from Annelida. This result was based on outgroup choices such as Mollusca and Sipuncula, and may well be misleading. Alternative hypotheses are therefore worth outlining, though they do not follow normal cladistic practice. Storch , following a detailed study on the musculature of Annelida, proposed that scale-worms, a diverse clade within Phyllodocida, are representative of the plesiomorphic condition for Annelida.

He suggested that there was a radiation from this group, but that Chrysopetalidae were most closely related to scale-worms.

The implication of his hypothesis is that Phyllodocida represents a paraphyletic group, from which all other polychaete taxa arise.

Westheide and see Westheide et al. This would either result in a paraphyletic Phyllodocida or Amphinomida, depending on which taxon is used as the root Fig. The conflict between this molecular sequence data and the morphological results could be caused by several factors. Further morphological study, combined with sequence data, may uncover these 'losses' see Hessling and Westheide, However, the molecular sequence data sets assembled to date have been marked by both a limited number of taxa and characters.

An exception is Brown et al. They recovered some morphological groupings such as Cirratulidae, Terebellidae and Eunicida, but did not show a monophyletic Phyllodocida or Aciculata, nor did they find any parts of these taxa to be basal groups of Annelida.

However, some expected groupings were not recovered. None of the major taxa used here, such as Palpata, Aciculata, Phyllodocida, Canalipalpata, Sabellida or Terebellida were recovered in Martin's analysis. Also less diverse taxa such as Nereididae, Spionidae and Aphroditiformia were not recovered. In a review of the fossil record of annelids Rouse and Pleijel suggested that the oldest unequivocal fossil polychaetes, such as Canadia from the Cambrian, belong within Phyllodocida.

Figure 1. The clitellum, seen here as a protruding segment with different coloration than the rest of the body, is a structure that aids in annelid reproduction. Annelids display bilateral symmetry and are worm-like in overall morphology. Annelids have a segmented body plan wherein the internal and external morphological features are repeated in each body segment.

This metamerism is thought to arise from identical teloblast cells in the embryonic stage, which give rise to identical mesodermal structures. The overall body can be divided into head, body, and pygidium or tail. The clitellum is a reproductive structure that generates mucus that aids in sperm transfer and gives rise to a cocoon within which fertilization occurs; it appears as a fused band in the anterior third of the animal Figure 1.

The epidermis is protected by an acellular, external cuticle, but this is much thinner than the cuticle found in the ecdysozoans and does not require periodic shedding for growth. Circular as well as longitudinal muscles are located interior to the epidermis. Annelids show the presence of a true coelom, derived from embryonic mesoderm and protostomy.

Hence, they are the most advanced worms. A well-developed and complete digestive system is present in earthworms oligochaetes with a mouth, muscular pharynx, esophagus, crop, and gizzard being present. The gizzard leads to the intestine and ends in an anal opening.

A cross-sectional view of a body segment of an earthworm a terrestrial type of annelid is shown in Figure 2; each segment is limited by a membranous septum that divides the coelomic cavity into a series of compartments.