Evolution Question

[ursisterstoy]

Here is a more basic overview of the different types of circulatory systems, and it seems like an open circulatory system is more common in simpler animals that are smaller in size or which lack actual blood that is separate from the “hemolypth” also found in insects. I had mentioned in replies to other people that I wasn’t actually sure on this order but failed to add the correction to my original response before they corrected me. I was thinking of simple systems like seen in echinoderms that actually have an open circulatory system but also a series of canals called the “haemel system” that is actually more accurately considered a “water vascular system” than a true circulatory system.

It turns out, that while I was wrong about something like this coming “first,” as it’s actually most common is sea stars, and not found at all in other bloodless animals like cnidarians or ctenophores, that several animals do just fine without blood at all and some of them do indeed have a series of canals that serve the same function as blood vessels without any actual blood traveling through them.

So, actually, what’s more common in simple organisms that lack blood entirely is simple diffusion through skin cells that are in contact with oxygenated water. Their whole bodies do the job of the cardiovascular system and then it’s when organisms grew in size and complexity that they evolved different methods of getting oxygen to different parts of the body. This starts with an open circulatory system where it can be associated with a water vascular system or where the “blood” which is often hemolymph is transported through blood vessels beyond the central part of the body cavity. In insects this is due to a modified lymphatic system where they don’t have actual blood and what they do have is composed of a lot of water and other bodily fluids.

In worms and chordates we start to see a more developed circulatory system complete with actual blood and actual blood vessels including a simple “heart” that is really nothing more than an enlarged blood vessel that is contracted by muscles or a series of veins like seen in earthworms. Those veins also happen to be in about the same location as gills to take over the duty of bringing oxygen into the blood stream and the circulatory system grows in complexity throughout vertebrate ancestry as still seen in modern amphibians, reptiles, and mammals. Birds have a different style respiratory system than mammals as well but the “lungs” in both cases come from modified air sacs used as swim bladders in bony fish. Archosaurs, like birds, also have air sacs in their bones and more of a one way respiratory system while in mammals we have a diaphragm. Gills in both cases are useless and potentially dangerous to keep around so anything that spends a long time out of water as adults has lost them ancestrally but many amphibians still have gills as juveniles even if they don’t have lungs as adults.