A. Protists, Sponges and Cnidaria

1. Protists--the ciliates as an example

a. Paramecium has two contractile vacuoles, one located near the gullet and the other at the opposite end. The system has rays leading into the contractile vacuole. These fill up and then squeeze their contents into the contractile which in turn empties its contents to the exterior.

b. It is not completely known what causes the contractile vacuoles to contract other than ATP is associated with the process, as in the presence of an ATPase, nothing happens

c. Rate of contract of the two vacuoles differs, with the one near the gullet contracting more often.

d. Other protists--they also have contractile vacuoles which are used to get rid of surplus fluid.

2. Sponges, cnidaria etc.

a. Since no cell in any of these organism is too far away from the medium surrounding the body, all can excrete ammonia directly into the environment.

b. There is not much of a problem with water balance in marine organisms, as they are in osmotic balance. There is more of a problem in freshwater organisms, as they are out of osmotic balance and are constantly taking in water that they need to get rid of.

B. Platyhelminthes

1. Flame cells- Flagellated cells at the end of the excretory canals.

2. Called flame cells by early microscopists because through the faulty lens systems of their microscopes, the beating flagellae at the end of the flame cell looked like a small "fire".

3. Assumed relationship between these acoelomate animals and their solid parenchyma and the need for a system to remove wastes from this solid mass of cells. Cells of epidermal layer can excrete ammonia directly to the exterior.

C. Nematodes

1. Two lateral line cells that have an excretory canal in their middles.

2. As nematodes have a constant cell number, as they grow, the cells simply get larger. The excretory system consists of these two cells that run the length of the animal with a cross branch that empties into an excretory pore.

3. The fluid of the pseudocoel bathes one part of these cells and it is assumed that materials diffuse from the pseudocoel fluid into the canal in the middle of the cell.

D. Annelids

1. Paired nephridia in each segment.

2. Three functions to a nephridium

a. Extraction of wastes from the coelomic fluid--a ciliated funnel in the preceding segment

b. Differential retention of materials--a convoluted tubule in the middle section of the nephridium, often associated with a capillary bed.

c. A storage section--place where material can not only be stored, but where there might be additional reabsorption of water before getting rid of the "urine".

d. This pattern is retain by most other larger invertebrates with complex excretory systems.

E. Mollusks

1. Cephalopods--Because they consume foods with lots of proteins, one might expect them to have a more serious excretory problem. This is solved by a very effective excretory system with structures that have a direct attachment to the central circulatory system and good supply of blood.

2. Other mollusks--Perhaps less efficient excretory systems because of less active life styles. Other mollusks have a more open circulatory system with mixing of blood and lymph fluids (haemolymph).

F. Arthropods.

1. Green gland of malacostracan crustaceans.

a. Has the three components of the annelid, that is segment that drains haemolymph, segment that excretes excess nitrogenous wastes and reaborbs other materials, and then a storage component (bladder) that stores urinary materials.

b. In lobsters and crayfish, these paired glands are in the anterior body cavity at the base of the antennae. A small canal joins the bladder to an excretory pore at the base of the antennae.

2. Malphigian tubule system of insects

a. A series of tubules at the junction of the mid-gut and hind gut. They excrete uric acid crystals directly into the lumen. An interesting pattern associated with how the material becomes more solid and water is reabsorbed.

b. This relates to the space between the lining of the hind gut and the osmotic pressures of the material in the tubules and hind gut.

G. Echinoderms

1. The use of dermobranchiae not only for respiratory purposes, but also for excretion. These structures are extensions from the coelomic cavity that penetrate through the endoskelton and form small thin ballon-like projections from the epidermis. The fluids in the dermobranchiae can directly release ammonia and other materials into the fluid bathing these "skin gills".

H. Some general thoughts.--In organisms having a more complex structure for the excretory system, there is a great deal of metabolic energy spent in making the system function. Sometimes, systems such as the gills also aid in excretion, particularly ionic regulation. For example many ions are excreted by the gill epithelial cells. This seems sensible because of the flow of media over the gills that not only would bring ions to the gills for possible absorption, but also could take them away.