Phylum Annelida

Body plan Annelids are coelomate animals (meaning they have true coelom. Even if this reduced secondarily.) They normally have long thin bodies composed of a series of identical segments. These segments lie between the head, comprised of prostomium, a mouth sometimes a perisotium, and a tail called a pigydium.

Habitat Most species prefer aquatic environment.

Organs Body has more than two cell layers, tissues and organs. Body cavity is true coelom, often divided by internal septa. Body possesses a through gut with mouth and anus. Body possesses 3 separate sections, a prosomium, a trunk and a pigydium. Has a nervous system with an anterior nerve ring, ganglia and a ventral nerve chord. Has a true closed circulatory system. Has no true respiratory organs.

Methods of reproduction Reproduction is sexual or asexual. Asexual reproduction is by fragmentation, budding, or fission. Sexual reproduction reproducing annelids hermaphrodites are common, but most species have separate sexes.

Examples of annelids

Major Classification Polychaeta, Oligochaeta (earthworms) and Hirudinea (leeches). The familiar annelids for most people are earthworms and leeches. Pelagic polychaetes that swim or drift, preying on other plankton. Few groups occurring in fresh water and moist terrestrial surroundings. The overall systematic of the group remain stable.

Evolution Recent cladistic studies suggest that the phylum Annelida is not a monophyletic grouping and that polychaetes arose from a metameric ancestor independently of the oligochaetes and leeches. Some authorities unite the Annelida and the Arthropoda under a taxon originally established by Cuvier called Articulata. Most systematists view the polychaetes as ancestral annelids, but details of annelid origin are debated.

General features Annelids are similar to roundworms. They are bilaterally symmetrical. Have three germ layers. Tube with in the tube body plan and organs. True coelom and segmentation which both allow for greater specification of body parts.

Some important information… They do not have many specialized sense organs. They filter the blood and excrete wastes out the excretory pore. Their food enters the mouth into the pharynx. Their circulatory system consists of a dorsal and ventral blood vessels.

Mode of Nutrition The digestive tract of oligochaetes is tubular and straight. The mouth leads to muscular pharynx acts as a pump for ingesting food. They also help regulate the pH of body fluids. The intestine is a straight tube and is the principal site of digestion and absorption, A dorsal fold of the lumenal epithelium called the typhlosole substantially increase the surface area of the intestine.

Works cited annelida.htmlhttp:// annelida.html E1-annelida.htmlhttp:// E1-annelida.html