The annelids include earthworms, polychaete worms, and leeches. All members of the group are segmented, in other words, made up of segments. Segmentation is also called metamerism, which is a linear sequence of body segments. Segments each contain elements of such body systems as circulatory, nervous, and excretory tracts. Metamerism increases the way the body moves by allowing the effect of muscle contraction to be extremely relaxed, and it makes possible the development of greater mobility in general body organization. The phylum Annelida (segmented worms) is diverse, containing the earthworms, leeches, and marine worms. They are all triploblastic (containing three different segments) and bilaterally symmetrical. In addition, annelids have a body wall with both longitudinal and circular muscle layers,a complete digestive tract, a nervous system, a closed circulatory system, and an excretory system.
There are three major classes of Annelida:
Class Polychaeta- mostly marine worms or clam worms
Class Hirudinea- the leeches (mostly freshwater)
Class Oligochaeta- moslty freshwater and terrestrial worms like earthworms.
There are three major classes of Annelida:
Class Polychaeta- mostly marine worms or clam worms
Class Hirudinea- the leeches (mostly freshwater)
Class Oligochaeta- moslty freshwater and terrestrial worms like earthworms.
Class Polychaeta
Clam worms filter feed using cilia. They catch food in sticky mucus and move it into the mouth via cilia. However, they only retain some ability to filter-feed and rely on their bacterial symbionts that live within their gills for the major source of their nutrition. The symbionts receive nutrients and oxygen, which flow into the clam through the uppermost end of the shell.
Class Oligochaeta
Most earthworms are scavengers that feed on dead organic matter. They feed by passing soil through the gut, from which nourishment is extracted, or by eating organic debris, including leaves accumulated on the surface of the soil. The digestive system is divided into a number of regions, each with a special function. Food that enters the mouth is swallowed by the action of the muscular pharynx, then passes through a narrow esophagus that has three swellings on each side. These are the calciferous glands that excrete to dispose of excess calcium obtained in the food. The food then moves to the crop, which seems to serve only as a storage organ, and then to the muscular gizzard. With the aid of very tiny stones swallowed by the worm, the gizzard grinds the food thoroughly. Food is then digested by juices secreted by gland cells in the intestine.
Class Hirudinea
Hemophagic (feeding on blood) leeches attach to their host and remain there until they become full, at which point they fall off to digest. They all have an anterior (oral) sucker formed from the first six segments of their body, which is used to connect to a host for feeding, and can also release an anesthetic to prevent the host from noticing the leech. They use a combination of mucus and suction to stay attached. Many leeches have a proboscis used for swallowing the prey or for sucking its fluids and some others have jaws for biting. Many parasitic leeches are able to parasitize a wide variety of hosts. Most of the marine and some of the freshwater leeches are fish parasites. The medicinal leech, Hirudo medicinalis, is one of a group of aquatic bloodsucking leeches with jaws. Another group of jawed bloodsuckers is terrestrial, these leeches live in damp tropical vegetation and drop onto their mammalian prey. Most parasitic leeches attach to the host only while feeding, a single meal may be up to 5 or 10 times the weight of the leech and provide it with food for several months. The digestive tract of bloodsuckers produces an anticoagulant, hirudin, which is something that keeps the blood from clotting.