- Phylum name:
- Annelida
- Also called:
- segmented worm
- Related Topics:
- beard worm
- leech
- earthworm
- polychaete
- oligochaete
- On the Web:
- The Canadian Encyclopedia - Annelida (Mar. 05, 2025)
The nervous system of free-moving polychaetes is similar to that of oligochaetes. It consists of a dorsal brain, or supraesophageal ganglion, which is a discrete mass of nervous tissue in the prostomium; a pair of nerves united ventrally to form the ventral subesophageal ganglion; and paired nerve cords with one ganglion per segment. In sedentary polychaetes, the brain may become highly modified.
The muscles of annelids are coordinated both by the ventral nerve cord, which is composed of two strands and extends the length of the worm, and by a ganglion and nerves located within each segment. The nerves within each segment carry impulses away from the ganglion (motor nerves) or toward it from a sensory receptor (sensory nerves). The cell bodies of sensory nerves are located beneath the surface epithelium; those of motor nerves are either within the ganglion or in separate parapodial ganglia. Each segmental nerve innervates those components of the body wall, parapodia, and the digestive tract found in its segment.
The nerve cord of many annelids has giant nerve fibres (neurochords), which may have either a simple or a compound structure. Simple neurochords are very large single nerve cells; their axons arise from cells situated in either the brain or a segmental ganglion. Compound neurochords are multiple structures; each axon is connected to numerous cell bodies along its course. The function of the giant nerve cord is the rapid transmission of impulses from one end of the worm to the other; this enables the longitudinal muscles of each segment to contract at about the same time. The value of rapid contraction is evident in the escape reaction of tube-dwelling sedentary polychaetes.
Some giant nerve fibres convey impulses as fast as vertebrate nerve fibres (about 21 metres per second); annelid fibres, however, are larger in diameter (1.5 millimetres in Myxicola) and lack a thick insulating sheath (myelin). Not only is recovery from the passage of impulses slower in giant nerve fibres than in other annelid nerves but the former are also the last component to develop in the nerve cord of a growing worm. The nerve cord of Myxicola contains one giant nerve fibre, which is used to study the properties of the nerve impulse. In Myxicola, an impulse may be conducted in either direction along the nerve, unlike Nereis or the earthworm; may be initiated at any level; and is an all-or-none action.
Digestive system
The polychaete digestive system is generally a straight tube; a mouth leads into an esophagus, which is followed by the intestine and the anus. Some free-moving forms have a proboscis that can be thrust forward by being turned inside out—that is, the proboscis is eversible. In oligochaetes such as the earthworm, the mouth opens into a muscular pharynx, which opens to the esophagus and then to a muscular gizzard. The intestine, which extends most of the length of the worm, terminates in an anus. In leeches, the mouth, surrounded by the anterior sucker, opens into the esophagus; the crop and intestine follow—each with minute pockets (diverticula)—then the rectum and anus.
Most annelids, except leeches, either lack or have poorly developed diverticula, minute pockets that serve as digestive glands. Instead, the gut lining contains secretory cells (concentrated in the foregut) and absorptive cells (concentrated in the hindgut). Digestive enzymes are most active in the gut. Digestion within cells has not been demonstrated in annelids. A lengthwise fold, the typhlosole, hangs downward in the intestinal cavity of oligochaetes. The absorptive surfaces of the typhlosole and of the anterior intestine may have a brush border; fats are absorbed only in this region.
Calciferous glands, found only in certain earthworms, apparently excrete calcium by secreting granules of calcium carbonate that are transformed into calcite crystals in the intestine.
Excretory system
The basic units of the annelid excretory system are either protonephridia, which have tubules (solenocytes) that end blindly within cells, contain flagella (whiplike projections), and are joined to a common duct that drains to the outside; or metanephridia, which are funnel-shaped structures containing cilia (short, hairlike processes) that open to the outside.
Ammonia is the chief nitrogen-containing end product of protein metabolism in aquatic annelids; earthworms, adapted to living in the soil, excrete more of another nitrogen-containing compound, urea, probably as part of a mechanism to control salt and water balance in the worm. The sea mouse Aphrodita, a polychaete, excretes 80 percent of its nitrogen as ammonia, which is also the primary nitrogenous excretory product in leeches (smaller amounts of urea also are excreted). Part of the ammonia excreted by leeches may come from bacteria in part of the leech’s excretory system (nephridial capsules). The ability of leeches to withstand high concentrations of ammonia is believed to result from a protective effect provided by high levels of calcium in their cells.
Three aspects of nephridial function in annelids correspond to those of the vertebrate kidney—filtration, resorption, and secretion. Coelomic fluid filters through solenocytes. The ciliated funnels of metanephridia retain minute particles and those of moderate size. In oligochaetes, whose coelomic fluid contains proteins, particles are actively absorbed in the ciliated region of the tubule. The tubules of earthworms also resorb inorganic ions such as sodium and calcium and can selectively eliminate excretory products from both the coelomic fluid and the bloodstream.
Respiratory system
Gas exchange generally takes place through the skin, but it may occur through gill filaments in some polychaetes or through the rectum of aquatic oligochaetes. Although oxygen may be transported directly in the blood, it is usually carried by a respiratory pigment, either hemoglobin or chlorocruorin. Hemoglobin, the most common pigment, is present in most free-moving and some sedentary polychaetes and in most oligochaetes and leeches. Chlorocruorin is found in several polychaete groups (Flabelligerida, Terebellomorpha, and Serpulimorpha). A few free-moving polychaetes, some oligochaetes, and rhynchobdellid leeches have colourless blood. The blood of the polychaete Serpula vermicularis contains both pigments, the young having more hemoglobin and the old more chlorocruorin.
Annelid hemoglobin molecules have several properties in common with the hemoglobin found in vertebrates but differ in molecular weight and in the relative amounts of certain constituents. Chlorocruorin differs from hemoglobin in having a lower affinity for oxygen and in being green in dilute solutions, red in concentrated ones.
The properties of annelid respiratory pigments are associated with the mode of life of the worm. The hemoglobin of the lugworm Arenicola, a polychaete, releases oxygen to the tissues only under conditions of extreme oxygen deficiency. The hemoglobin of some earthworms takes up oxygen from a normal atmosphere but releases it only when tissue oxygen is low and, thus, may protect the worm from oxygen poisoning.
Circulatory system
The circulatory system in the lower oligochaetes consists of a dorsal vessel that arises from a blood sinus or capillary network surrounding the intestine and conveys blood forward; a ventral vessel that conveys blood backward; and connective vessels between the two. The blood vessel walls consist of an outer membranous (peritoneal) layer containing muscle fibres, a middle region of collagenous material, and an inner lining of thin cells (endothelium). In higher oligochaetes, one or more pairs of hearts connect the dorsal and ventral vessels and propel the blood. In free-moving polychaetes the dorsal vessel is the chief propulsive force, and networks of small vessels connect the dorsal and ventral ones. In some leeches the blood is propelled by a dorsal vessel connected by loops at both ends to a ventral one.
Blood is moved by wavelike contractions of the blood vessels, by the beating of cilia, or by pumping provided by hearts. In Arenicola and the earthworm the heartbeat apparently is initiated in nerve cells rather than in muscle tissue, as occurs in vertebrates. The blood apparently carries nitrogen-containing products to the nephridia for excretion. The only blood cells are amoebocytes, which are free-moving cells that engulf particles.
Hormones
The brain contains several types of cells whose secretory activities relate to phases of the life cycle, especially those of reproduction, growth, and regeneration.
Neurosecretory cells, which are nerve cells that produce hormones, are found in the brain; their structure, similar to that of nonsecretory nerve cells, consists of fine projections (an axon and neurofibrils) and a cell body. The secretions of neurosecretory cells, which terminate in the walls of a blood vessel, in other fluid systems, or in the epidermis, are in the form of microscopic droplets or granules. Neurosecretory cells seem to be derived from epidermal secretory cells that have been incorporated into the central nervous system.
Inhibitor hormones are known in some Phyllodocida, and a stimulator substance has been identified in Drilomorpha, both of which are polychaete groups. (For a discussion of inhibitor hormones in nereids and syllids, see above Reproduction.) The maturation of gametes is apparently inhibited in nephtyid polychaetes by neurosecretions of the brain. The brain of the lugworm Arenicola stimulates maturation of gametes.
The brain has been shown to play a role in the regeneration of the posterior end of the body of polychaetes such as nereids and nephtyids, but the effect may be an indirect one involving the genital inhibiting hormone. Neurosecretory cells occur in the brain and subesophageal ganglia of several terrestrial and aquatic oligochaete species. Removal of the brain from sexually maturing earthworms causes degeneration of the clitellum and prevents gamete formation. The brain also plays a role in osmoregulation, as indicated by the increase in chloride concentration in the urine of oligochaetes lacking a brain. The neurosecretory cells in the brain of leeches control gamete formation.
Evolution and paleontology
The annelids are considered to have evolved in the sea, perhaps from an ancestral flatworm that evolved through the trochophore larva, the characteristic early stage of polychaetes. The oligochaetes are thought to have developed from polychaete stock; the leeches, which have the clitellum in common with the oligochaetes, probably evolved from the latter.
The question of which polychaete order preceded the others remains unresolved. The Archiannelida were long considered to have been the earliest polychaete group because of their primitive condition; however, some members (e.g., Polygordius) that lack setae and external segmentation and have simple nervous, muscular, and circulatory systems are now considered to be a specialized group. Polygordius species typically are small in size; they have cilia on their surfaces for locomotion, respire through the skin, and have internal fertilization. Finally, the larvae undergo non-pelagic development. The polychaetes appear therefore to have undergone radiative evolution, in which every character has been modified independently of the others. There is thus little basis for regarding any one order as ancestral to the others.
The evolution of oligochaetes from polychaetes may be related to the change from a marine to a freshwater habitat. One view is that oligochaetes evolved in marine swamps and were subjected to periodic drying; survival during dry periods would have been made possible by egg cocoons. A contrary hypothesis is that the primitive oligochaete was adapted to permanent freshwater conditions rather than to a terrestrial habitat. Some authorities consider the oligochaetes to have evolved from some members of the order Eunicida (e.g., the family Lumbrineridae) or the order Capitellida (e.g., the family Capitellidae), but this may result from a superficial resemblance in body form and thus may be of little evolutionary significance.
Reproductive structures provide not only the main criteria for understanding the course of evolution within the oligochaetes, but the basis for the classification of oligochaetes as well.
Each of the oligochaete orders, Lumbriculida, Monilogastrida, and Haplotaxida, is considered to have evolved separately from primitive oligochaetes. Many, however, believe that two paths of evolution occurred. In one pathway, the vas deferens (the tube carrying sperm from the testes) opened outward on the segment immediately behind the segment that contains the testes and evolved into two lines differentiated on the basis of whether the seminal receptacle (a storage cavity) opened in front of the testes, or at the same segment, or posterior to the testes. In the second principal pathway, the vas deferens opened a few segments behind the testes.
There is little doubt that the leeches evolved from the primitive oligochaetes, since both groups have a clitellum, at least during the reproductive period, and both are hermaphroditic. The Acanthobdellae are considered to be the link between the oligochaetes and leeches because they possess setae and walls between segments; the order contains only one known species, however. The three remaining orders of leeches evolved into two lines based on whether or not the animals have jaws.
The fossil record of annelids is limited because they have almost no hard body parts. Tubes constructed by polychaetes and polychaete jaws are the most commonly encountered fossil specimens. Most fossil records of oligochaetes are doubtful, and fossil leeches are unknown. Some burrows, or tubes, have been interpreted as belonging to wormlike creatures from Precambrian strata (more than 620,000,000 years old). Fossils resembling the scale worm Halosydna and the sea mouse Aphrodita, Nereis-like forms, and calcareous tubes similar to present-day Serpula and Spirorbis species have been described. The shells of Paleozoic mollusks (more than 230,000,000 years old) are occasionally marked by U-shaped tubes similar to those made by the polychaete Polydora, a modern-day pest of oysters. The tough jaws of polychaetes, containing minute spiny black teeth known as scolecodonts, occur from the Cambrian Period (about 570,000,000 to 500,000,000 years ago) onward.
