Annelids: The Segmented Worms
Classes
Class Polychaeta: the Polychaetes, mostly marine worms (this is the one we will focus on)
Class Hirudine: the Leeches
Class Oligochaeta: the terrestrial worms (like earthworms)
Class Polychaeta: the Polychaetes, mostly marine worms (this is the one we will focus on)
Class Hirudine: the Leeches
Class Oligochaeta: the terrestrial worms (like earthworms)
Anatomy
Annelids are all triploblastic (meaning they have an ectoderm, mesoderm, and endoderm), they are bilaterally symmetrical, and eucoelomate (having a "true" coelom or body cavity).
All annelids have 3 segments: a prosomium, a trunk and a pygidium. Each segment contains a nervous, circulatory, excretory and muscular system. Oxygen and Carbon Dioxide will diffuse through the skin. Segments also have parapodia (made up of setae) that increase the surface area of the skin. They are also sometime used as "legs" to move the worm.
Nervous System
Annelids have a brain or cerebral ganglion connected a the ventral nerve cord. The ventral nerve cord, is usually made up of a pair of cords that runs the length of the body; however, in most polychates the two cords are fused.
Locomotion
Annelids have circular and longitudinal muscles which in combination with their parapodia allow for a wide range of movement.
Circulatory System
A closed circulatory system is present in most polychaetes. In smaller polychaetes, a circulatory system may be absent.
Digestive System
Annelids have a one tract digestive system that is basically a tube straight from mouth to anus. Annelids have nephridium (plural nephridia). This is an invertebrate organ which basically acts as the kidney. The nephridia occur in pairs and filter out metabolic wastes before excretion.
Some polychaetes are carnivorous while others eat sand/mud digesting the bacteria & nutrients found in the sediment.
Reproduction
Most polychaetes reproduce sexually. This can occur with separate sexes or they can be hermaphroditic. Those worms that can reproduce asexually are usually capable of sexual reproduction too.
Once the egg has been fertilized the larvae may remain in the water column as long as a couple of months or as short as a few hours. Once it settles to the sea floor it will undergo metamorphosis to the intermediate form before developing into an adult.
Annelids are all triploblastic (meaning they have an ectoderm, mesoderm, and endoderm), they are bilaterally symmetrical, and eucoelomate (having a "true" coelom or body cavity).
All annelids have 3 segments: a prosomium, a trunk and a pygidium. Each segment contains a nervous, circulatory, excretory and muscular system. Oxygen and Carbon Dioxide will diffuse through the skin. Segments also have parapodia (made up of setae) that increase the surface area of the skin. They are also sometime used as "legs" to move the worm.
Nervous System
Annelids have a brain or cerebral ganglion connected a the ventral nerve cord. The ventral nerve cord, is usually made up of a pair of cords that runs the length of the body; however, in most polychates the two cords are fused.
Locomotion
Annelids have circular and longitudinal muscles which in combination with their parapodia allow for a wide range of movement.
Circulatory System
A closed circulatory system is present in most polychaetes. In smaller polychaetes, a circulatory system may be absent.
Digestive System
Annelids have a one tract digestive system that is basically a tube straight from mouth to anus. Annelids have nephridium (plural nephridia). This is an invertebrate organ which basically acts as the kidney. The nephridia occur in pairs and filter out metabolic wastes before excretion.
Some polychaetes are carnivorous while others eat sand/mud digesting the bacteria & nutrients found in the sediment.
Reproduction
Most polychaetes reproduce sexually. This can occur with separate sexes or they can be hermaphroditic. Those worms that can reproduce asexually are usually capable of sexual reproduction too.
Once the egg has been fertilized the larvae may remain in the water column as long as a couple of months or as short as a few hours. Once it settles to the sea floor it will undergo metamorphosis to the intermediate form before developing into an adult.