The metanephridia of annelids process coelomic fluid

Filtration of body fluids and modification of urine by tubules are highly developed processes in annelid worms, such as the earthworm. Recall that annelids are segmented and have a fluid-filled body cavity, called a coelom, in each segment (see Figure 32.22). Annelids have a closed circulatory system through which blood is pumped under pressure. The pressure causes the blood to be filtered across the thin, permeable capillary walls into the coelom. The cells and large protein molecules of the blood stay behind in the capillaries, while water and small molecules leave them and enter the coelom. In addition, some waste products, such as ammonia, diffuse directly from the tissues into the coelom. But where does this coelomic fluid go?

Each segment of the earthworm contains a pair of metanephridia (singular, metanephridium; from the Greek meta, "akin to," and nephros, "kidney"). Each metanephrid-ium begins in one segment as a ciliated, funnel-like opening in the coelom, called a nephrostome, which leads into a tubule in the next segment. The tubule ends in a pore, called the nephridiopore, that opens to the outside of the animal (Figure 51.5). Coelomic fluid enters the metanephridia through the nephrostomes. As the fluid passes through the tubules, their cells actively resorb certain molecules from it and actively secrete other molecules into it. What leaves the animal through the nephridiopores is a hypotonic (dilute) urine containing nitrogenous wastes, among other solutes.

^ The tubule cells of the metanephridium alter the composition of coelomic fluid...

^ The tubule cells of the metanephridium alter the composition of coelomic fluid...


51.5 Metanephridia in Earthworms The metanephridia of annelids are arranged segmentally.The cross section (at the left) shows a pair of metanephridia. Longitudinal sections (at the right) show only one metanephridium of the two in each segment.

Was this article helpful?

0 -2
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


  • kiros negassi
    How are metanephridia arranged in annelids?
    8 years ago
  • uffo
    How are metaneprhridia arranged in annelids?
    8 years ago
  • emilie
    How are metanephridium arranged in annelids?
    8 years ago
  • luigia
    How are metanephridia arranged in annelid?
    8 years ago
  • kirsti
    What is coelomic fluid?
    8 years ago
  • isa
    What is the process of metanephridia?
    8 years ago
  • mike
    How does a metanephridium takes up fluid?
    8 years ago
  • abraham nuguse
    How does water enters coelom?
    8 years ago
  • Franziska Hofmann
    How are the origin and end of metanephridium arranged in relation to the segments?
    8 years ago
  • fethawit
    How are the nephrostome and nephridiopore arranged?
    8 years ago
  • feaven
    What are nephrostomes in annelids?
    8 years ago
  • raija
    What body fluid is directly filtered into the tubules of metanephridia?
    7 years ago

Post a comment