The hard outer coverings of some sea urchins, called 'tests', allow local deformation that may resist impact loading by incorporating collagen-swathed sutures.

           
  "Some of the few relatively large shells with thin walls are those of sea urchins and other echinoid echinoderms. They resemble pressure-supported structures…but they lack the requisite internal pressures (Ellers and Telford 1992), so they have to have proper shells, at least in the engineering sense. For the biologist, they have 'tests' rather than 'shells,' and the latter distinction isn't just our usual terminological proliferation. Tests, unlike shells, are growing structures of articulated hard elements. For some, at least, collagen-swathed sutures permit significant local deformation, which should reduce impact loading and thus offset some of the hazards of a thin shell (Telford 1985). Nonetheless, they do smash easily…The best rationalization I can offer for why sea urchins tolerate such fragility is that the wave forces don't provide either piercing loads or a sudden hammering impact." (Vogel 2003:388)
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The shell of sea urchins prevent cracking and breaking via oblate shape.

     
  "Spheres are also distorted by gravity. If a drop of liquid, held together by surface tension, is placed on a surface and therefore subjected to the force of gravity, it tends to become a more flattened shape, called 'oblate'. The shell of a sea urchin, stripped of its spines, is oblate. This shape distributes stress evenly over the surface and therefore reduces the likelihood of cracking or breaking. The guiding principle of economy is always apparent: a shape is most efficient when it reduces its work to a minimum." (Foy and Oxford Scientific Films 1982:20)
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Sea urchins minimize the distance materials must be transported from a central point due to their radiating shape.

     
  "If you let a drop of ink fall on a piece of paper, the splash pattern that results looks rather like a sea urchin. If you drop water into a bowl of liquid and photograph the moment of impact with high-speed equipment, the coronet shape formed at the surface resembles a sea anemone. Yet another of the basic shapes of life - the explosion, or radiating shape - repeats the forms taken by falling drops of water. Radiating shapes occur wherever numerous lines fan outwards from a single central point - whether in a flat plane, as with a starfish, or in three dimensions, as with a sea urchin. The plant kingdom is full of radiating shapes: the majority of flowers have this form, and many plants grow leaves that radiate directly from a stem base; but there are many examples in the animal kingdom as well. Radiating lines, as a construction design, have two useful attributes: they minimize the distance between the centre and the outlying points, and they provide great scope for increasing the surface area of an organism…The first of these qualities is most convenient in cases where materials must be transported rapidly from the centre to outer points or vice versa. There is a disadvantage, however. If there are a lot of outlying points, the lines tend to become overcrowded around the centre (diagram a). One way to overcome this problem is to develop branching patterns, to reduce the total length of travel and the congestion of lines at the centre (diagram b). If each artery and vein in the body led directly to the heart, for example, the heart would be swamped in a vast tangle of blood vessels. Instead, a few large central vessels divide and redivide into smaller branches. Physically, the resistance to flow or skeletal strength are reduced when the vessels coalesce or the skeletal rays are fused. Biologically, the smaller branching vessels help animals survive damage and aid their development and growth." (Foy and Oxford Scientific Films 1982:24)
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There are approximately 940 species of echinoids distributed worldwide in marine habitats from the intertidal to 5000 meters deep. Their fossil record is extensive due to their test (an internal skeleton), and dates back to the middle Ordovician period.

 

Echinoids are commonly grouped as regular or irregular, with the greatest differences pertaining to the oral structure, shape of the organism, and location of the anus. Regular echinoids are the sea urchins; they are generally found on rocky substrates. Irregular echinoids are the sand dollars, which are generally found on sandy or soft ground.

 

Like all echinoderms, echinoids are pentaradially symmetrical, have a water-vascular system, and have an internal skeleton made of calcitic ossicles (plates). A distinguishing feature of the echinoids is that the ossicles imbricate (overlap) and are fused into a globular or discoidal test; its flattened or concave oral side faces the substratum and the aboral side is arched in most species. The mouth, in the peristomal membrane, contains a powerful chewing apparatus called the Aristotle's lantern. The lantern is composed of five jaws and is capable of extending through the mouth of some urchins. The mouth leads to the intestine and anus, which is located in the center of the aboral surface in regular echinoids. The anus is either posterior or on the oral surface of irregular echinoids.

 

Spines and tube feet surrounding the peristome function in locomotion, burrowing, and food-gathering. Generally, urchins have longer spines; sand dollars have shorter spines which give them a fuzzy appearance. Tube feet are a part of the water vascular system characteristic of all echinoderms. Pincers located between spines are called pedicellariae. Some types of pedicellariae and specialized spines of urchins contain venom used in self-defense.

 

In regular urchins, the ossicles, or plates, of the test are arrayed in ten longitudinally oriented columns. Two adjacent columns each form one of five ambulacral series. These are the plates through which tube feet extend. On the aboral side, the tube feet function in respiration and sensation. The ambulacral series of plates are conspicuous in the cleaned test of a sand dollar: restricted to the aboral side, they are arrayed in a petaloid pattern. At the aboral end of the interambulacral series of regular urchins are the five (sometimes four) genital plates, through which the gonopores open. One of the genital plates serves as the sieve plate, or madreporite, for the water vascular system. Together the madreporite, the anus, and the gonopores make up the periproct.

 

Most echinoids have five conspicuous gonads arrayed interambulacrally. The sexes are separate. In some species, gametogenesis is regulated by photoperiod so that spawning of most or all members of a population occurs during the same time. Some female urchins brood their young externally, within the protection of their spines or tube feet. In species with indirect development, an echinopluteus larva is produced. Such a larva is bilaterally symmetrical, and undergoes metamorphosis to attain the pentaradial symmetry of the adult.

 

Echinoids graze on just about anything they come across, plant or animal. This includes algae, bryozoans, and dead animals.

 

Members of this class are food for crabs, sea stars, fish, birds, otters, and other mammals. Probably the single most important contribution of these animals to scientific knowledge is their embryological development. Researchers investigate the development of deuterostomes using sea urchin eggs, due the clear radial cleavage during a zygote's development. Echinoids of economic importance for the U.S. are the red (Strongylocentrotus franciscanus), the purple (S. purpuratus), and the green (S. droebachiensis) sea urchins. These urchins are harvested for their roe and are exported to Japan; the roe, called uni, is used in sushi.

 

  References: 

 

Brusca, R.C. and G.J. Brusca. 1990. Chapter 22: Phylum Echinodermata. Invertebrates. Sinauer Associates, Inc., Sunderland, Massachusetts.

 

Hyman, L.H. 1955. The Invertebrates: Echinodermata: The Coelomate Bilateria. Volume IV. McGraw-Hill Book Company, Inc. New York and other cities.

 

Kozloff, E.N. 1990. Chapter 21: Phylum Echinodermata. Invertebrates. Saunders College Publishing. Philadelphia and other cities.

 

Price, R.J. and P.D. Tom 1995. Sea urchins. Sea Grant Extension Program Publication. http://seaurchin.org/Sea-Grant-Urchins.html