Echinoderms: Sea Stars And Their Relatives (Rev, 1987)

e if you explore tide pools along the rocky Sea Coast an animal you are almost certain to find is the starfish or sea Star the sea stars are members of a group of ocean dwelling creatures called echinoderms which also include sand dollars and sea urchins echinoderms live on the sea floor on rocks sand or mud and the group is found nearly everywhere in the ocean one of the features that distinguishes eadms from other animals is the Stony spines plates and bumps that cover their bodies in fact the word eodm means spiny skinned some scientists think this prickly armor helps protect these rather sluggish animals from predators let's take a closer look at the eadm body plan using the sea star as an example of the group all adult eadms have radial symmetry this means their bodies are organized like a wheel with spoke surrounding a central Hub typically eadms have five-part symmetry and in SE Stars this shows up as five or more arms extending from a central disc each arm is hollow and contains reproductive and digestive organs the spines that cover the eodm body project from an underlying network of hard Limestone plates the spines such as these and the plates that lie embedded in the flesh give the animal a supporting skeleton in some eadms the skeleton is rigid and shell-like but in sea stars and others it's flexible and the body can twist into many positions respiration is carried on by skin gills these fleshy bumps distributed among the spines oxygen diffuses through the skin gills directly into into the fluid-filled body cavity tiny pincer-like spines catch and remove any debris or small animals that might interfere with the delicate skin gills sea stars and other eadms have a unique method of locomotion their movement depends on water pressure generated by a network of canals called the water vascular system seawater enters the system through an opening called the Civ plate and flows through canals that extend into the arms to rows of tube feet when a muscular sack at the top of each tube foot contracts water is forced forward and these suckers extend and attach to the surface when the water is pushed back into the sack the tube foot contracts and the animal is pulled forward the tube feet of the sea star lie in grooves that are guarded by rows of movable spines these spines can be closed over to provide protection the sea star has no brain but it is able to coordinate its movements by means of nerves that Circle the central disc and extend into the arms at the tip of each arm is a patch of light sensitive cells called the eye spot most eadms react to strong changes in light although they are slow moving the Sears have little trouble catching their principal food clams oysters muscles and other B valve mollusks but not all B valves sit still for the star this scallop for instance uses shell capping jet propulsion to make a quick Escape Most B valves however aren't nearly as mobile when a sea star captures a clam it grips the clam with its arms while the suckers on its tube feet hold tightly to the Shell the sea star then uses the hydraulic pressure of its water vascular system to force open the clam when there's a slight Gap in the shells something incredible happens the sea star pushes its stomach out of its mouth through the Gap and into the body of the clam the clam is digested in its own shell and the sea star absorbs the nutrients through its extended stomach All That Remains is an empty clam shell While most sea stars have five arms some species have six some 10 or 11 or even 21 arms sometimes sea stars will lose an arm but they have remarkable powers of regeneration within months this missing arm will be completely replaced other members of the class of eadms are the sea cucumbers of these animals the characteristic eodm spines are reduced to microscopic size plates embedded in the leathery skin and the five-part symmetry of the kerms occurs as five bands of tube feet these are two of the bands in some types of C cucumbers all five bands are found on one side of the body and the animal creeps forward on its tube feet in the same manner as a sea star although eadms vary greatly in appearance they all have tube feet and spiny skin in the sea urchin for instance the spines have become long and exaggerated making the creature look a bit like an animated pin cushion like their spines urchin tube feet are also very long and slender underlying the spines Limestone plates have fused to form a solid globe-shaped skeleton evenly spaced around the skeleton are five double rows of holes for the tube feet once again you can see the ecoderm characteristic five-part symmetry sea urchins feed on plants and animals either living or dead they have an elaborate chewing apparatus composed of five pointed teeth that Circle the mouth sand dollars look much like flattened sea urchins they use their short movable spines to bury themselves in the sand the last class of ecoderm are the serpent Stars you can see the snake likee arm motion that gives them their name these animals are also known as brittle stars because their arms drop off easily this individual is regenerating an arm the tube feet of serpent stars are very small and are used not for Locomotion but for their sense of touch like many Marine creatures eadms reproduce by releasing eggs and sperm directly into the seawater where fertilization takes place eadms are widely used in embryological research because they can be easily induced to shed their gtes this is especially true of sea urchins here a mild electric current causes a female to release her eggs in a male urchin white streams of sperm are released the eggs and sperm are collected with a pipet a drop or two of sperm mixed with the eggs is all that it takes to start the process of Sea Urchin Development urchins are able to pass through their entire embryological development in a simple Beaker of seawater magnified by a microscope you can see the sperm dark around the large spherical egg when the egg and a single sperm unite a fertilization envelope begins to rise off the egg surface after about an hour the newly fertilized egg under go its first division about 45 minutes later the two cells divide into four a few minutes later eight then 16 and so on after about a day of cell division a sea urchin embryo hatches from the fertilization envelope and swims about with beating Celia at this stage it is a hollow ball of cells called a blastula after a few more days of development the embryo has an opening that will become its mouth and gut it is now called a gastrula or little stomach next the embryo becomes a self-feeding larvae that drifts with the ocean currents it's only after weeks of feeding and growing that they settle to the bottom and undergo metamorphosis to become radially symmetrical adults echinoderms are among the most abundant and conspicuous bottom dwellers in the ocean realm for scientists they have become an important research animal and for all of us the eadms show The Grand diversity of form achieved by Nature sh