Sea urchin
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Sea urchins or urchins are the class Echinoidea within the echinoderms. Approximately 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal zone which actually is n
Paracentrotus lividus, a regular sea urchin (Euechinoidea, infraclass Carinacea)
A sand dollar, an irregular sea urchin (Irregularia)
Phyllacanthus imperialis, a cidaroid sea urchin (Cidaroidea)
Coelom
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The coelom is the main body cavity in many animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In oth
Classification of tripoblasts based on body cavities