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Annelida (segmented worms). By: Raymund Riegl. Characteristics of Annelids. Annelida means “little rings.” There are about 15,00 annelid species, ranging from less than 1 mm to the 3-m length. (giant Australian earthworm)
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Annelida (segmented worms) By: Raymund Riegl
Characteristics of Annelids • Annelida means “little rings.” • There are about 15,00 annelid species, ranging from less than 1 mm to the 3-m length. (giant Australian earthworm) • All members of the group are to some extent segmented. (a.k.a metamerism) This 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.
Annelids are schizocoelous. • 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, and digestive system. Gases are exchanged through the skin. Each segment typically contains a pair of nephridia.
Annelids may be monoecious or dioecious. Larva may or may not be present; if present they are of the trochophore type. Some forms also reproduce asexually. • Annelids have some combination of tactile organs, chemoreceptors, balance receptors, and photoreceptors. • The nervous system includes a pair of cephalic ganglia attached to double nerve cords that run the length of the animal along the ventral body wall.
Classifications of the Phylum • Oligochaeta (terrestrial and freshwater segmented worms; e.g., earthworms) • Polychaeta, (mostly marine segmented worms) sometimes referred to as bristle worms. Common representatives include the lugworm, and the sandworm. • Hirudinea, Commonly referred to as Leeches.
Morphology of Oligochaeta(earthworms) • Earthworm is the common name for the largest members of Oligochaeta. • Educed head; no parapodia, but setae present. • The basic body plan of an earthworm is a tube, the digestive system, within a tube, the muscular slimy, moist outer body. It has several specialized regions: the pharynx, the esophagus, the crop, the gizzard, and the intestine. • Earthworms have a simple circulatory system. A typical earthworm will have 5 hearts. Tiny blood vessels are abundant in the earthworm’s skin, which functions as its reparatory organ.
Morphology continued • A brain like pair of cerebral ganglia lies above and in front of the pharynx. A ring of nerves around and in front of the pharynx connects to a sub-pharyngeal ganglion, form which a fused pair of nerve cords run posteriorly.
Embryology of the earthworm • It will generate their earliest segments during larval life. Larval segments arise from a field of dividing cells located in lateral regions of the body. • The larva emerges as small, but fully formed earthworms, except for a lack of the sex structures, which develop later in about 60 to 90 days.
Life History • The worms hatch as smaller earthworms that lack reproductive organs; this is the main difference between newly hatched worms and the fully developed. The worms start to develop reproductive organs after they are hatched. They are able to use their reproductive organs at around 12 months. Earthworms can sometimes live for more than 10 years depending on the species.
Morphology of Polychaeta • Clam worms, sand worms, and tubeworms range in length from 0.078 inches to 9.8 feet. Their bodies consist of a head, body trunk, and tail. • Most species have long, segmented bodies that are tube like and covered with bristles. Along the sides of their bodies are flaps that help them to swim, burrow, draw bits of food suspended in the water to their mouths, and grip surrounding rocks, sand, or mud. • The side flaps are also used like gills to help them breathe underwater.
Morphology continued • They vary in color, ranging from clear to light tan, red, pink, green, yellow, or a combination of these and other colors. • Their body shapes also vary and usually reflect their lifestyles. Active species, such as those that hunt for their food and some burrowers, have bodies with segments that are all very similar in appearance to one another. • They have well-developed flap like appendages, eyes, and other sensory organs. Some of these species have a mouth with tough jaws and the ability to extend part of their digestive tract outside the body to feed.
Embryology • The embryo is transparent giving insight into the dynamic structures and processes inside the cells that accompany the prevailing anisotropic cleavages. • s they develop, additional segments are formed behind a special growth zone.
Live History • Their lifespan can range from a few weeks to several years. • A unique feature of this animal within the phylum is the shape of the Christmas-tree worm.
Morphology of Hirudinea • Fully mature adults can be up to 20cm in length. • Leeches have eyespots on the head that are able to detect movement from contrasting patterns of light and shadow. • Leeches are not covered with stiff bristles nor do they have paddle like flaps. • These organisms have two suckers, one at each end, called the anterior and posterior sucker. The posterior is mainly used for leverage while the anterior sucker, consisting of the jaw and teeth, is where the feeding takes place.
Morphology continued • The incision leaves a mark which is an inverted Y inside of a circle. • After piercing the skin and injecting anti-coagulants (Hirudin) and anaesthetics they suck out blood. Large adults can consume up to ten times their body weight in a single meal. • Leeches can live for up to a year between feeding.
Embryology • Once mating has occurred, The cocoons are either left in the soil, or attached to the bodies of other animals. • Fish leeches attach their egg cases to the bodies of crustaceans, or water-dwelling animals.
Life History • Leeches are capable of living 1 or more years. • It is a parasitic species. • It is disease causing when it is removed by burning.
Conclusion • All Annelida are segmented. • Live in water and or land. • Are basically elongated tubes that move. • Some may be parasitic and venomous
