New forum link: https://forum.biologyonline.com/index/evolution
1. Digestive cavity - in phyllum Porifera - intracelular digestion which realize through epithelium of choanocytes.
2. Digestive duffle - in orgs with diploblastic body plan - Cnidaria, Acnidaria (Coelenterata) - this has only one opening - mouth(thats why duffle), intracelular digestion changes to extracelular digestion.
3. Digestive tube - Bilaterally symetrical orgs - this has two openings - mouth and anus (thats why tube), theres extracelular digestion.
Theres a lot of exceptions. For example gastropoda has uni and extra together.
The most primitive animal phyla possess only a single body cavity, which typically has either digestive or circulatory functions, or both. There is no secondary body cavity, or coelom, and consequently these phyla are referred to as the acoelomates.
Most animal phyla, however, have evolved a second body cavity of one form or another. The pseudocoelomates, which include a number of worm-like phyla, are characterized by a secondary body cavity known as the pseudocoelom. The pseudocoelom has some but not all of the characteristics of true coeloms. Finally, several animal phyla, including those that possess the most complex body plans in the kingdom, are characterized by a body cavity known as a true coelom. These phyla are known as the eucoelomates.
here are some links:
Biology: Visualizing Life. New York: Holt, Rinehart and Winston, Inc., 1998.
Gilbert, Scott F. Developmental Biology, 5th ed. Sunderland, MA: Sinauer Associates, 1997.
Gould, James L., and William T. Keeton. Biological Science, 6th ed. New York: W. W. Norton, 1996.
Hildebrand, Milton, and Viola Hildebrand. Analysis of Vertebrate Structure. New York: John Wiley, 1994.
Karp, Gerald, and N. J. Berrill. Development. New York: McGraw-Hill, 1981.
Larson, William J. Human Embryology. New York: Churchill Livingstone, 1997.
Moore, Janet. An Introduction to the Invertebrates. Cambridge, U.K.: Cambridge University Press, 2001.
i was wondering about the same: how did the digestive system develop? I was looking through your explanations and I found that all you two did was give different names for different digestive systems and explain how they work. The most towards and explanation I read was:
Fransis wrote:In fact, the increasing complexity of animal form and function during the evolution of the group can be directly linked to the evolution of ever-more-sophisticated body cavities. . . . Most animal phyla, however, have evolved a second body cavity of one form or another. . . .
here are some links: . . .
Beyond this, i dont understand HOW the digestive system evolved. Also, how did stomach acid end up in the stomach? Commonly, the acid in stomachs is enough to kill cells. How did the cells in stomach evolve resistance to these chemicals through bile production? How did those cells then know to dump that material outside the body? I would appreciate if you could explain the process of this and what mechanics (punctuated equilibrium, divergent evolution, etc) of evolution are involved.
Reason: quotes fixed
One aspect is the 30 year time span it took to develop a cecal valve when placed in a different environment -something that in their kind of animal, is rare.
They developed different diet, dentition, jaws, skulls, social behaviours, and gut, in a time frame that defies conventional wisdom. It could be a different phenome without necessarily any mutations. This would be shown if they were to have changed over mostly all at once, rather than as mutations spreading through the population.
Another aspect of the lizard life is how they alter their island environment through eating fruits and spreading the plant forms and so provide themselves with more food.
Although your explanation possibly lends some merit (I only say possibly because I have not reviewed it in depth), I still do not understand how it is possible through evolution to allow stomach acids to exist within the digestive system which would kill the cells which house them. Also, i do not understand how those stomach cells could have evolved resistance to something that would kill them instantly. Also, how did those cells even come to know what acid is and that it is harmful and life threatening, especially since it would kill those unadapted cells on contact. How did the system then create a means for expelling waste?
In that way, it's not as if it's original development happened in that 30 years time span, nor in the same manner, did it appear this time. It's more like it was taken out of a "cold storage vault".
2.How did cells come to genetically adapt to not being oxidized?
By the way you ask the question, it seems like another obstacle that evolution has overcome. If it is an obstacle, I would like to know how evolution overcame that. Also, oxygen continues to exist in the world, and cells must still have that genetic adaptation in their genetic make up. If so, where is that genetic information? Also, How did the chemical slosh (from where prokaryote is formed) not manage to include dissolved oxygen in the various parts of the cell?
Cis or trans? That's what matters.
However, we can propose hypothesis based on evidences we have. But it's still better than believe in God for which you have no proof at all (besides your belief, which is sufficient for many, but yet they ask for more from others).
You are honest enough to consider your view as a hypothesis. I find that commendable and it is worthy of respect.
Now Just consider this aspect of the digestive system
Lysosomes and peroxisomes are often referred to as the garbage disposal system of a cell. Both organelles are somewhat spherical, bound by a single membrane, and rich in digestive enzymes, naturally occurring proteins that speed up biochemical processes. For example, lysosomes can contain more than three dozen enzymes for degrading proteins, nucleic acids, and certain sugars called polysaccharides. All of these enzymes work best at a low pH, reducing the risk that these enzymes will digest their own cell should they somehow escape from the lysosome. Here we can see the importance behind compartmentalization of the eukaryotic cell. The cell could not house such destructive enzymes if they were not contained in a membrane-bound system..
http://www.ncbi.nlm.nih.gov/About/prime ... _cell.html
Now to the question, which came first?
The membrane OR the Lysosomes and Peroxisomes.
Evolutionary theory in whatever form, cannot explain how this “chicken and egg” situation can come about. Yet living organisms are replete with these types of systems.
Purposeful design is a perfectly valid scientific concept and only purposeful design can explain it.
I don’t see why anyone has to profess a belief in a God to recognise design.
Many people are simply agnostic.
Why does it have to be either one or the other?