Download File

The annelids (from Latin anellus "little ring") are a large phylum (Annelida) of invertebrate animals,
comprising the segmented worms, including the well-known earthworms and leeches. There are about
15,000 known modern species of annelids.
Annelids are found in most wet environments and include many terrestrial, freshwater, and especially
marine species (such as the polychaetes), as well as some which are parasitic or mutualistic. They range
in length from under a millimeter to over three meters (the seep tube worm Lamellibrachia luymesi).
In addition to enhancing the diversity of nature, which brings so much joy to humans, the segmented
worms are ecologically and medically important. They are common in marine, freshwater, and
terrestrial environments, serving as part of the food chain, and helping to turn over the soil and
sediments. Some segmented worms have commercial use as bait for sport fishing or food for tropical
aquarium fish. The leech can be used medically, for example, in controlling swelling, as it produces
chemicals that can serve as an anesthetic and prevent blood coagulation
The three major groups of annelids are the polychaetes (largely marine annelids, with over 5,500
species); the oligochaetes (earthworms and freshwater worms, with over 3,000 species); and the
hirundinea (leeches, with about 500 species). However, biological classification of annelids can vary
widely among taxonomists.
Some consider there to be three classes of annelids: Polychaeta, Clitellata, and Aelosomata. The
Clitellata are then further divided into three or four subclasses: Oligochaeta (earthworms and
freshwater worms), Hirundinea (leeches), and Branchiobdella (about 150 species of small animals that
are largely parasites or commensals on crayfish), and sometimes Acanthobdellida (leech-like, temporary
parasite, which is also placed in Hirundinea in some classifications). Aelosomata includes small to very
small annelides, with about 25 known species. This taxonomy looks like the following:
Class Polychaeta
Class Aelosomata
Class Clitellata
Oligochaeta - earthworms and so forth
Branchiobdellida — small, largely parasites or commensuals
Hirudinea - leeches
Acanthobdellida (sometimes part of Hirudinea)
Another taxonomic scheme regards two groups of polychaetes—the Archiannelida and the
Myzostomaria—as classes in their own right, and recognizes four total classes: Polychaeta, Clitellata,
Myzostomida, and Archiannelida. This looks like the following:
Class Polychaeta
Class Clitellata
Oligochaeta
Branchiobdellida
Hirudinea
Class Myzostomida
Class Archiannelida
In some biological classifications, the Clitellata is considered a subphylum and the Oligochaeta,
Hirudinea, and Branchiobdellida are treated as classes of this subphylusm.
A simple classification scheme is to recognize two classes of annelids, the Polychaeta and the Clitellata:
Clitellata Oligochaeta - The class Oligochaeta includes the megadriles (earthworms), which are both
aquatic and terrestrial, and the microdrile families such as tubificids, which include many marine
members as well.
Leeches (Hirudinea) - These include both bloodsucking external parasites and predators of small
invertebrates.
Polychaeta - This is the largest group of annelids and the majority are marine. All segments are identical,
each with a pair of parapodia. The parapodia are used for swimming, burrowing, and the creation of a
feeding current.
There have also been proposals to consider the Clitellata as part of the Polychaeta, thus making the
latter term synonymous with the annelids.
Annelids are triploblastic protostomes with a coelom (at least historically), closed circulatory system,
and true segmentation. Protosomes are animals with bilaterial symmetry where the first opening in
development, the blastophore, becomes its mouth. Triploblastic means that they have three primary
tissue areas formed during embryogenesis. A coelom is a fluid-filled body cavity.
Polychaeta: "A variety of marine worms" plate from Das Meer by M. J. Schleiden (1804–1881)
Oligochaetes and polychaetes typically have spacious coeloms; in leeches, the coelom is largely filled in
with tissue and reduced to a system of narrow canals; archiannelids may lack the coelom entirely. The
coelom is divided into a sequence of compartments by walls called septa. In the most general forms,
each compartment corresponds to a single segment of the body, which also includes a portion of the
nervous and (closed) circulatory systems, allowing it to function relatively independently. Each segment
is marked externally by one or more rings, called annuli. Each segment also has an outer layer of circular
muscle underneath a thin cuticle and epidermis, and a system of longitudinal muscles. In earthworms,
the longitudinal muscles are strengthened by collagenous lamellae; the leeches have a double layer of
muscles between the outer circulars and inner longitudinals. In most forms, they also carry a varying
number of bristles, called setae, and among the polychaetes a pair of appendages, called parapodia.
Anterior to the true segments lies the prostomium and peristomium, which carries the mouth, and
posterior to them lies the pygidium, where the anus is located. The digestive tract is quite variable but is
usually specialized. For example, in some groups (notably most earthworms) it has a typhlosole (internal
fold of the intestine or intestine inner wall), to increase surface area, along much of its length.
Different species of annelids have a wide variety of diets, including active and passive hunters,
scavengers, filter feeders, direct deposit feeders that simply ingest the sediments, and blood-suckers.
The vascular system and the nervous system are separate from the digestive tract. The vascular system
includes a dorsal vessel conveying the blood toward the front of the worm, and a ventral longitudinal
vessel that conveys the blood in the opposite direction. The two systems are connected by a vascular
sinus and by lateral vessels of various kinds, including in the true earthworms, capillaries on the body
wall.
The nervous system has a solid, ventral nerve cord from which lateral nerves arise in each segment.
Every segment has an autonomy; however, they unite to perform as a single body for functions such as
locomotion. Growth in many groups occurs by replication of individual segmental units; in others, the
number of segments is fixed in early development.
Reproduction
Depending upon the species, annelids can reproduce both sexually and asexually.
Asexual reproduction
Asexual reproduction by fission is a method used by some annelids and allows them to reproduce
quickly. The posterior part of the body breaks off and forms a new individual. The position of the break
is usually determined by an epidermal growth. Lumbriculus and Aulophorus, for example, are known to
reproduce by the body breaking into such fragments. Many other taxa (such as most earthworms)
cannot reproduce this way, though they have varying abilities to regrow amputated segments.
Sexual reproduction
Sexual reproduction allows a species to better adapt to its environment. Some annelida species are
hermaphroditic, while others have distinct sexes.
Most polychaete worms have separate males and females and external fertilization. The earliest larval
stage, which is lost in some groups, is a ciliated trochophore, similar to those found in other phyla. The
animal then begins to develop its segments, one after another, until it reaches its adult size.
Earthworms and other oligochaetes, as well as the leeches, are hermaphroditic and mate periodically
throughout the year in favored environmental conditions. They mate by copulation. Two worms, which
are attracted by each other's secretions, lay their bodies together with their heads pointing in opposite
directions. The fluid is transferred from the male pore to the other worm. Different methods of sperm
transference have been observed in different genera, and may involve internal spermathecae (sperm
storing chambers) or spermatophores that are attached to the outside of the other worm's body. The
clitellata lack the free-living ciliated trochophore larvae present in the polychaetes, the embryonic
worms developing in a fluid-filled "cocoon" secreted by the clitellum.
Fossil record
The annelid fossil record is sparse, but a few definite forms are known as early as the Cambrian. There
are some signs they may have been around in the later Precambrian. Because the creatures have soft
bodies, fossilization is an especially rare event. The best-preserved and oldest annelid fossils come from
Cambrian Lagerstätten, such as the Burgess Shale of Canada, and the Middle Cambrian strata of the
House Range in Utah. The annelids are also diversely represented in the Pennsylvanian-age Mazon Creek
fauna of Illinois.
Relationships
The arthropods and their kin have long been considered the closest relatives of the annelids on account
of their common segmented structure. However, a number of differences between the two groups
suggest this may be convergent evolution rather than a feature passed on by common descent.
The other major phylum that is of definite relation to the annelids is the mollusk, which shares with
them the presence of trochophore larvae. Annelids and Mollusks are thus united as the Trochozoa, a
taxon more strongly supported by molecular evidence