The Theme of Structure and Function in Animal Nutrition and Digestion
Variation on a Theme
We learned all about the amazing nephrons in the vertebrate kidney, but what happens in the invertebrates? These guys don't have kidneys or nephrons, but they still need to regulate how much water they keep and how much waste they get rid of. Each animal's body has to have some sort of mechanism to take in or expel water, and keep things balanced.
Since many invertebrates are more primitive, understanding how these animals regulate ion concentrations, conserve water, and excrete wastes gives us a unique glimpse into the evolutionary processes underlying osmoregulation and vertebrate kidney function.
If you are a freshwater sponge (and perhaps live in a pineapple under the sea), you'll use contractile vacuoles for osmoregulation. These are little pouches that gather water from inside the cell depending on its osmotic state, and then contract to push water out of the cell. The speed that the water fills the vacuoles and get expelled changes. Since they physically move water from one place to another, contractile vacuoles also regulate cellular volume.
If you happen to be a flatworm, things are a bit different. You'll probably use a type of nephridial organ. Since the word nephridial shares a root with nephron, it basically means that the nephridial organ functions sort of like a nephron. Genius, we know. This nephridial organ is a super basic nephron with dead-end tubules. At the dead ends are terminal cells, and within the terminal are cilia (picture hair blowing in the wind) whose movement encourages wastes to leave the animal and enter the tubule. Other small molecules can filter into the tubule too, but bigger stuff stays put. This waste-laden solute then passes through the nephridial organ and exits through a pore on the animal's surface. It's sort of like they are sweating urine. How's that for a mental image?
Mollusks have a similar, albeit more developed, osmoregulating system. These animals have longer tubules that are open at both ends and allow for a decent amount of reabsorption of nutrients back into the blood. There's even an area for fluid storage (which is similar to our bladders).
Crustaceans, like lobsters and crabs, use a totally awesome excretory system called antennal glands. Yep, their excretory system actually uses their antennae! At the base of an antenna is a gland that has an end sac, a bladder, a twisting tubule, and an excretory pore. When water and other ions enter the antennae, they end up interfacing with blood vessels (imagine a vertebrate glomerulus), wastes get secreted, and important ions get reabsorbed back into the blood. This coiled excretory tubule ends with a pore at the base of the antennae, where water and waste are removed. For those keeping score at home, we've now discussed creatures that basically sweat urine, and ones that expel waste from the front of their face. Be glad to be a human.
Since grasshoppers and other insects are more developed creatures, their excretory and osmoregulating systems look relatively similar to ours. They use malpighian tubules, a mess of twists and turns that are somewhat permeable to ions, water, and waste. Once these tubules connect to the midgut and hindgut, a lot of the filtrate gets reabsorbed back into the blood, and urine becomes concentrated since water conservation is a must. Their solid urine then "flows" out of a rectum-like organ.
We've just discussed a bunch of different ways for living creatures to get their waste out of their bodies, but comparing them, the basic fundamental principle is the same. Gather up the waste and water that's gone through the system, recover whatever salvageable water or ions there are, and get the rest of it out of there.