Lisa Peczenyj, ape 10, of Allentown, Pennsylvania, for her question:
Does the clam have a heart?
The clam likes to bury himself in sandy or muddy beaches. Tie wears two saucer¬shaped shells and when fully dressed one of the small type clams can nestle in the hollow of your hand. Yes, he has a heart. But as you mould expect, it must be a very small one. It also is less complicated than the big human heart. But the clam's simple little heart works just fine for his simple little body.
The heart of a clam or any other creature is somewhat like a fist made of sturdy muscle. It Yorks like a pump to circulate nourishing blood among the body's busy living cells. A single celled animal, such as the ameba, does not need a heart. Many slightly larger creatures have more cells arranged to two simplified layers. They do not need hearts either because their nourishing fluids can seep from cell to cell.
Naturally, none of these midgets have backbones or bones of any sort. Neither do a lot of much larger creatures, such as clams, earthworms and insects. However, their bodies are made from multitudes of cells organized to do special work, such as digesting food. These creatures need pumping hearts to circulate oxygen and nutrients throughout their bodies.
The simplest heart is a bulge in a blood vessel that fills and empties as it pumps. An earthworm has several pairs of these hearts, More advanced animals have two, three, or four compartments, or chambers inside their hearts. Host fishes have two chambers, frogs and turtles have three. Birds, mammals and humans have four chambered hearts.
You might expect the clam to have the simplest kind of heart. After all, he is not advanced enough to have a bony skeleton like the fishes. But, actually, his tiny heart has three tiny chambers. However, his colorless blood successfully carries dissolved gases and floating particles of nourishment among his busy cells.
As in more advanced animals, the clam's heart works on a team together with his stomach, his intestines and his gills. His internal organs are held together in a soft package which is wrapped in a loose fleshy flap called the mantle. His gills are elaborate pockets on the inside of his mantle: They are stuffed with tiny blood vessels and washed with streams of water that flog in and out through the partly opened shells. The same streaming crater also serves scraps of food to the clam's little mouth.
His pumping heart circulates the blood through a network of vessels, pores and pockets. Along the way, the blood gathers digested food and fresh oxygen from the gills and delivers them to the cells.
The tiny heart is folded around the clam's intestine and wrapped in a soft sack of fluid. It pumps the blood through arteries in two directions, forward and backward. The forward stream is pushed through the internal organs and also through the clam's muscular foot. The backward stream is pushed through the fleshy mantle. There some of it seeps through pores and fine vessels that lead to and from the dills.