Common colors include black and dull shades of green, olive, brown, purple, blue, and red. There is a wide diversity of shapes an introduction to embryology balinsky pdf free download sea urchins. Sea cucumbers and the irregular echinoids have secondarily evolved diverse shapes.
This is most apparent in the “regular” sea urchins, which have roughly spherical bodies with five equally sized parts radiating out from their central axes. Several sea urchins, however, including the sand dollars, are oval in shape, with distinct front and rear ends, giving them a degree of bilateral symmetry. In these urchins, the upper surface of the body is slightly domed, but the underside is flat, while the sides are devoid of tube feet. This “irregular” body form has evolved to allow the animals to burrow through sand or other soft materials. The lower half of a sea urchin’s body is referred to as the oral surface, because it contains the mouth, while the upper half is the aboral surface. The test is rigid, and divides into five ambulacral grooves separated by five interambulacral areas. Each of these areas consists of two rows of plates, so the sea urchin test includes 20 rows of plates in total.
The plates are covered in rounded tubercles, to which the spines are attached. The mouth lies in the centre of the oral surface in regular urchins, or towards one end in irregular urchins. It is surrounded by lips of softer tissue, with numerous small, bony pieces embedded in it. This area, called the peristome, also includes five pairs of modified tube feet and, in many species, five pairs of gills. The structure of the mouth and teeth have been found to be so efficient at grasping and grinding that their structure has been tested for use in real-world applications. The fully formed spicule is composed of a single crystal with an unusual morphology. In other echinoderms, the endoskeleton is associated with a layer of muscle that allows the animal to move its arms or other body parts.