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Description
My submission for COM #53 (s1.zetaboards.com/Conceptual_E… ). This picture shows the anatomy of an imaginary transitional species between turtles and bivalvian mollusks.

«Ever since their appearance back in the Triassic, turtles have been very unusual vertebrates. Their ribcage expanded into a shell that enclosed the soft parts of the body and surrounded the thoracic and pelvic girdles, leading to an almost arthropod-like body structure. In some species of the human era, papillae in the cloaca can absorb oxygen from the water, acting as a secundary respiratory system.
And yet, this is nothing compared to the changes that some of these creatures went through after that. A certain lineage of turtles moved to the floor of rivers, spending a long time embedded in the mud, waiting for a passing prey. Their reptilian metabolic rate allowed them to survive for a long time without resurfacing to breathe, but eventually the cloacal papillae took over the lungs as the main breathing organ. Eventually, they developed into a feathery gill (ctenidium) while the cloaca enlarged so that feces didn't interfere too much with breathing.
At this point, the mud-turtles were free to spend their entire life on the bottom of rivers. At a certain point, they developed a semi-mature phase that was able to mate and swim, so that the females could retain fertilized eggs for all their adult life; but then the males started producing hardy gametes that were able to spread in the water, and the females to release eggs on the floor around them.
Now that the mud-turtles' lifestyle required no movement at all, it also required little food. They could survive simply by feeding on food particles carried by the stream, catching them in the open mouth. Most of the muscles greatly reduced. In this age, the greatest danger for them was a species of eel-like carnivore fish that could bite the head and the limbs even when they were retracted in the shell. The limbs, by then all but useless, were eventually lost, and their opening was completely closed by bone. The cranium fused with the shell, and only the tongue remained motile.
Water filtration became internal. Lungs, no more useful for breathing, were used to pump water in and out the digestive tract: the pharynx formed two tubes (siphons) that could reach outside. Eggs became smaller, more numerous and more buoyant, and they hatched into tiny free-swimming larvae. Since the energy need was so low, even the circulatory system degraded until only the heart and the aorta remained, and hemoglobin was lost as well.
This is when the most dramatic transformation occurred. The mud-turtles turned on their side, while the shell split on the opposite one. The shell turned into two hinged valves, one derived from the carapace, the other from the plastron. The valves could open along what was once the side of the turtle, allowing water to flow into the old cloaca (pallial cavity) to carry oxygen to the gill, and to take away waste and gametes. By this time, the only structures that could get out of the shell were the double siphon and the tongue, which was now large and strong enough to move the entire body around in a bounding or crawling motion whenever the necessity arised.
(This stage is depicted below.)
Eventually the internal skeleton disappeared, and the nervous system simplified into a cord of ganglia. The descendants of the mud-turtles had regained their motility, though in a way completely unlike any other vertebrate; they survived adequately on filtering water, and they even moved out in the ocean. The shell changed from its old scaly appearance to a ringed structure that allowed a faster growth, starting from the hinge; the siphons moved along the digestive tube towards the pallial cavity.
They had nothing in common with turtles, or even vertebrates, anymore. They had become clams.»

Description
My submission for COM #53 (s1.zetaboards.com/Conceptual_E… ). This picture shows the anatomy of an imaginary transitional species between turtles and bivalvian mollusks.

«Ever since their appearance back in the Triassic, turtles have been very unusual vertebrates. Their ribcage expanded into a shell that enclosed the soft parts of the body and surrounded the thoracic and pelvic girdles, leading to an almost arthropod-like body structure. In some species of the human era, papillae in the cloaca can absorb oxygen from the water, acting as a secundary respiratory system.
And yet, this is nothing compared to the changes that some of these creatures went through after that. A certain lineage of turtles moved to the floor of rivers, spending a long time embedded in the mud, waiting for a passing prey. Their reptilian metabolic rate allowed them to survive for a long time without resurfacing to breathe, but eventually the cloacal papillae took over the lungs as the main breathing organ. Eventually, they developed into a feathery gill (ctenidium) while the cloaca enlarged so that feces didn't interfere too much with breathing.
At this point, the mud-turtles were free to spend their entire life on the bottom of rivers. At a certain point, they developed a semi-mature phase that was able to mate and swim, so that the females could retain fertilized eggs for all their adult life; but then the males started producing hardy gametes that were able to spread in the water, and the females to release eggs on the floor around them.
Now that the mud-turtles' lifestyle required no movement at all, it also required little food. They could survive simply by feeding on food particles carried by the stream, catching them in the open mouth. Most of the muscles greatly reduced. In this age, the greatest danger for them was a species of eel-like carnivore fish that could bite the head and the limbs even when they were retracted in the shell. The limbs, by then all but useless, were eventually lost, and their opening was completely closed by bone. The cranium fused with the shell, and only the tongue remained motile.
Water filtration became internal. Lungs, no more useful for breathing, were used to pump water in and out the digestive tract: the pharynx formed two tubes (siphons) that could reach outside. Eggs became smaller, more numerous and more buoyant, and they hatched into tiny free-swimming larvae. Since the energy need was so low, even the circulatory system degraded until only the heart and the aorta remained, and hemoglobin was lost as well.
This is when the most dramatic transformation occurred. The mud-turtles turned on their side, while the shell split on the opposite one. The shell turned into two hinged valves, one derived from the carapace, the other from the plastron. The valves could open along what was once the side of the turtle, allowing water to flow into the old cloaca (pallial cavity) to carry oxygen to the gill, and to take away waste and gametes. By this time, the only structures that could get out of the shell were the double siphon and the tongue, which was now large and strong enough to move the entire body around in a bounding or crawling motion whenever the necessity arised.
(This stage is depicted below.)
Eventually the internal skeleton disappeared, and the nervous system simplified into a cord of ganglia. The descendants of the mud-turtles had regained their motility, though in a way completely unlike any other vertebrate; they survived adequately on filtering water, and they even moved out in the ocean. The shell changed from its old scaly appearance to a ringed structure that allowed a faster growth, starting from the hinge; the siphons moved along the digestive tube towards the pallial cavity.
They had nothing in common with turtles, or even vertebrates, anymore. They had become clams.»