The First Many Celled Animals: Sponges (rev, 1988)

if you look closely at a drop of pond water what you'll see is a vision of life on Earth as it looked 1 billion years ago this is an amoeba and cells much like these were probably the first animals to evolve on our planet the single celled animals that swarmed in Earth's ancient oceans were small and not one of them evolved to a size larger than a microscopic spec of protoplasm instead what eventually appeared was this an organized mass of many cells a multi-celled animal it had the simplest of body plans no head no mouth no digestive or circulatory systems just a few cell types working together to perform the essentials of life you might be wondering why such a simplistic multi-celled animal evolved in the first place why didn't single celled animals like the amoeba just evolve to a larger size the answer it turns out has more to do with physics than biology let's say that this Cube represents a cell and that these ingoing arrows represent oxygen entering the cell you can see that oxygen can only enter through the exposed surface of the cell once inside the oxygen reaches the center of the cell by diffusion these outgoing arrows represent waste leaving the cell if a cell became too large diffusion would eventually be unable to carry in oxygen and Carry Out waste fast enough the solution to the problem is to divide the large mass into smaller units what this does is increase the exposed surface area and so allow a greater amount of diffusion here's a way to measure just how much surface area can be gained this large cube is 3 in on a side 27 of the small cubes equals the size of the one large Cube this paper covers a cube much like a membrane surrounds a cell when you lay it out flat you can see the surface area of the large Cube peeling the paper off the small cubes you can see the enormous increase in surface area that's gained by subdividing the larger Mass this is the reason single celled animals did not evolve to be larger they couldn't their volume increased faster than their ability to move oxygen and waste by diffusion the only way large animals could be built was to bring together a body of small cells the early multi-celled animals were very simple but they were also very successful in fact their descendants still Thrive today and we call them [Music] sponges most SP es live in Shallow seas and fresh water although some have been found at depths of nearly 21,000 ft they range in size from half an inch to 6t across and range in color from black and white to red yellow green and purple they can be shaped like balls discs vases or branching trees and although adult sponges tend to stay attached to objects sometimes the objects can move the more than 5,000 species of sponges make up the phm porifera which means Poe bearers that's a fitting name because a sponge body resembles a Sac filled with holes sponges gather food by drawing in water through these holes the water passes into a main Central cavity and then exits by way of a large excurrent opening using a d we can watch the flow of water through the body of a living sponge in through the porous walls and out through the large xcurrent opening if the dye had been particles of food the sponge would have retained them how to answer that we have to look at the way a sponge is constructed the outside of a sponge body is protected by a layer of covering cells which fit snugly together like the tiles in a mosaic projecting through this layer are Poe cells where water flows into the animal lining the inner cavity are collar cells they're called collar cells because of the delicate sheet of cytoplasm that circles their long flagellum it's the synchronous beating of these fella that pulls a current of water through the sponge here's where the particles of food that are carried by the water are captured stuck to the layer of collar cells between the covering cells and the collar cells are wandering mezen cells one of their main jobs is to take food from the collar cells and distribute it to other parts of the sponge another job of the mezen Kim cells is to secrete needles of calcium carbonate called spicules when magnified spicules look like a Thicket of interlocking spikes in the body wall of the sponge the spicules act as a kind of skeleton a rigid framework that gives the body shape not all sponges have skeleton made of calcium carbonate spicules some have skeletons made of glassy silica like this Venus's Flower Basket sponge still others are made of a fibrous protein called [Music] spongin These are the sponges that are still collected by commercial [Music] divers under a microscope you can see the dense network of Spong and fibers spongin is absorbent because water can be held in the pockets and pours between the fibers besides making spicules the mezen Kim cells are also involved in sexual reproduction in sponges some cells enlarge with reserves of food and become eggs others become sperm the sperm are carried out of the sponge by the water currents and into other sponges in the same way once it's fertilized an egg like this one develops into a swimming larv that escapes from its parent and wanders in search of a place to [Music] attach eventually it settles and grows into a young sponge besides sexual reproduction there's also a special type of asexual reproduction in some saltwater and all freshwater [Music] sponges mezen Kim cells and other cells group together to form a spore-like mass called a gemu gules have food stores coated in a thick layer of Stony spicules and they are very tough the living cells inside can withstand freezing and drying when conditions are just right the sponge cells emerge from the gemu coat they Clump together and grow into a new [Music] sponge the final way sponges reproduce is by asexual butt Ing and branching if a branch is torn free it can grow into a new complete [Music] individual the sponge's remarkable ability to regenerate is second to none among animals and it has long been an important research interest to [Music] science if you take a piece of living sponge and reduce it to a cloud of separate cells by squeezing it through a silk cloth something amazing happens the cells settle to the bottom of the container and begin to move about bumping and jostling one another after a while clumps form isolated stragglers join the groups and groups Join one [Music] another in about two weeks young sponges as large as a pee are formed it is the ultimate form of regeneration individual cells have crept together to remake the body science is interested in sponge regeneration because it may give us insights on wound healing tissue grafting and even organ transplant imagine being able to grow a severed limb or a failed organ early naturalists saw the sponge's branching body and their immobility and classified them as plants it wasn't until the mid 1800s that scientists realized that sponges were actually animals an ancient lineage that descended from the first multi-celled animals on [Music] Earth for