GENERAL HISTOLOGY

Detailed explanation-1: -Their outer skeleton–called a test–is made up of ten fused plates that encircle the the sea urchin like the slices of an orange. Every other section has holes through which the sea urchin can extend its tubed feet.

Detailed explanation-2: -Echinoids are marine animals belonging to the phylum Echinodermata and the class Echinoidea. They have a hard shell (referred to as a test) covered with small knobs (tubercles) to which spines are attached in living echinoids. The test and spines are the parts normally found as fossils.

Detailed explanation-3: -The test of sea urchins is made of calcium carbonate, strengthened by a framework of calcite monocrystals, in a characteristic “stereomic” structure. These two ingredients provide sea urchins with a great solidity and a moderate weight, as well as the capacity to regenerate the mesh from the cuticle.

Detailed explanation-4: -All sea urchins have a hard skeleton made out of calcium, called a test, under their spines. Biologists are able to identify the kind of urchin just by looking at its test. Sea cucumbers: Sea cucumbers have a mouth on one end of its body. They may look like a worm but they hold a lot of water in their body.