Phylum Annelida

Leeches

General Information Annelids are ringed, segmented worms Examples include earthworms and leeches Habitat: terrestrial, fresh water, marine Have bilateral symmetry with 3 body layers (ectoderm, mesoderm, & endoderm)

3 Unique Features Have a true coelom (body cavity) lined by mesoderm Have externally visible body segmentation Have a closed circulatory system

Coelom Located between the body wall and the intestine Lined by mesoderm Filled with fluid Acts as a buffer between body muscles and the intestine (allows worm to move without putting pressure on the intestine)

Body Segmentation The body is divided into segments Each segment contains a portion of the coelomic cavity Each segment is separated internally by septa (membranes)

Class Polychaeta Polychaete worms Marine Flattened bodies Body segments with paired paddle-like appendages (parapodia)

Class Oligochaeta Earthworms Have a few hairs Small head for burrowing May have a saddle-like swelling (clitellum)

Class Hirudinea Leeches Often aquatic Bodies with suckers Many are ectoparasites feeding on blood

Class Oligocheatea Earthworms will be our representative organism for this Phylum

Earthworm Ingestion Earthworms have a complete digestive system (one way digestive tract) Food (organic material in the soil) enters through the mouth The pharynx draws food into the mouth (suck in and swallows food) Food moves through the esophagus to the crop where it is stored temporarily

Earthworm Digestion Food moves from the crop to the gizzard Gizzard contains sand grains which help grind up food by muscular contractions Food moves into the intestine where enzymes chemically digest (break down) food into particles small enough to be absorbed into blood Intestine has a typhlosole which increases the surface area for nutrient absorption

Feeding and Digestion

Earthworm Elimination Undigested food waste moves to the end of the intestine and exits through the anus

Earthworm Respiration Gas exchange (O2 & CO2) happens via diffusion through the moist skin Mucus glands secrete mucus to keep skin moist O2 diffuses across the moist skin and into the capillaries and blood O2 is carried by hemoglobin (Hb) in the blood

Earthworm Circulation Earthworms have a closed circulatory system (blood is always contained in vessels) Closed circulatory systems allow for more effective delivery of nutrients which allows for increased activity and size Earthworms have 5 pairs of hearts which pump blood through the vessels

Earthworm Excretion Excretory organs called nephridia filter excess H2O and nitrogenous wastes from the coelomic cavity of one segment This waste passes through a long tube surrounded by blood vessels (much like our kidneys) and eventually exits through an excretory pore in the next segment Excess water is reabsorbed into the blood vessels

Earthworm Sexual Reproduction Earthworms are hermaphrodites (male & female on same individual) 2 earthworms cross-fertilize (exchange sperm) Earthworms have a swelling called a clitellum which secretes a mucus cocoon into which fertilized eggs are deposited

Earthworm Asexual Reproduction Earthworms are able to regenerate missing body parts

Earthworm Nervous System Earthworms have a paired ventral nerve cord which swells into ganglia (a mass of nerve cell bodies acting as a primitive brain) at the head

Earthworm Locomotion Uses longitudinal and circular muscles combined with the fluid pressure in the coelom and the setae (bristles) to help burrow and move Contraction of the circular muscles of anterior (front) cause worm to lengthen Paired setae (bristles) anchor worm to soil Longitudinal muscles contract causing worm to shorten pulling it’s posterior (tail end forward)

Evolutionary Significance Segmentation! Body segmentation lead to the ability to have specialized structures in the body plan of organisms. Share common ancestor with the Arthropods Closed Circulatory System – heart(s) and blood vessels True Coelom – mesoderm lines the digestive tract