Segmented worms were the first animals to display segmentation in their body plans. You can read about segmentation on the animal characteristics page. Each repeated segment has very similar features, but many are specialized for certain functions. Two thirds of annelid species live in the ocean where they originally evolves over 530 million years ago. They range in size from microscopic to over ten feet long!
Segments
Having a true coelom gives annelids fluid filled space that can contain specialized organs. Each segment of annelids contains digestive, excretory, and locomotor organs. Some segments are more specialized for specific functions. For instance, towards the anterior end of these worms are cerebral ganglia, or primitive brains. This brain is connected to a nerve cord that runs the length of the worm's body. Each segment is separated by internal body walls called septa. Nerve signals and nutrients pass through these through the nerve cord and circulatory system respectively. |
Specialized Organs
In a closed circulatory system, hearts pump blood through the body and back to the heart, distributing materials in and out of the blood stream throughout the body. The other worm phyla exchange nutrients through their skin or the pseudocoelom fluid. Worms have a highly modified gut with portions specialized for different types of digestion and organs called nephridia in each segment for excretion. Most annelids have bristles on each segment called setae that give them traction when they crawl. Some also have fleshy appendages called parapodia which can be differently specialized for movement or respiration. |
Marine Worms
Most marine worms belong to the class Polychaeta and are the largest group of annelids. Many are beautiful and have fleshy paddle-like parapodia on each segment which can be used to swim, burrow, or crawl. They also increase the animals surface area allowing for more gas exchange with their environment. Many species borrow, using their setae to stick into substrate and their long muscles to push their segments into the ground. |
Leeches
Belonging to the class Hirudinea, leeches have a body that lacks parapodia and setae, meaning they are not internally segmented internally. Most are free-living, but some have evolved to become parasites. Species that do suck blood do so by having saliva that prevents blood from clotting and breaks up blood clots. Leeches have been used in medicine both for their ability to suck out "bad blood" and to treat blood clots. |
Earthworms
Earthworms belong to the class Oligochaeta. They have no parapodia and only a few setae on each segment. They have no head region or eyes, but do have a cerebral ganglion (brain) and have other sensory cells and organs at different parts of their bodies. They eat soil and decaying matter that, once it passes through their specialized gut, leaves nutrient rich soil in their castings.
Digestion
Soil enters through the earthworm's mouth using the muscles in the pharynx. Once through the pharynx, it goes through the esophagus and into a storage chamber called the crop. The soil then moves to the gizzard where it is crushed and broken down. This broken down soil then moves down the long intestine where the large surface area allows for diffusion of materials across the intestinal wall. What ever is not digested, passes out the worm's anus.
Soil enters through the earthworm's mouth using the muscles in the pharynx. Once through the pharynx, it goes through the esophagus and into a storage chamber called the crop. The soil then moves to the gizzard where it is crushed and broken down. This broken down soil then moves down the long intestine where the large surface area allows for diffusion of materials across the intestinal wall. What ever is not digested, passes out the worm's anus.
Movement
Earthworms have a hydrostatic skeleton from the presence of a coelom that supports each segment. Their circular muscles in each segment allow the worms to pull against each segment, causing the worm to elongate when they contract. Their longitudinal muscle span the whole body and cause the worm to bunch when they contract. Earthworms use the setae to anchor their posterior segments into the ground then elongate their bodies. They push forward. Once elongated, they anchor setae in anterior segments and pull their bodies forward.
Earthworms have a hydrostatic skeleton from the presence of a coelom that supports each segment. Their circular muscles in each segment allow the worms to pull against each segment, causing the worm to elongate when they contract. Their longitudinal muscle span the whole body and cause the worm to bunch when they contract. Earthworms use the setae to anchor their posterior segments into the ground then elongate their bodies. They push forward. Once elongated, they anchor setae in anterior segments and pull their bodies forward.