Planets orbit the Sun while moons orbit around planets. The amount of time required for a planet or moon to complete one orbit around another body is called an orbital period. On Earth, we call this orbital period a year. Our year is 365.256 days. On Mars it takes 686.98 days (Earth days, that is) to complete one orbit about the Sun--that's a long time to wait between birthdays!
They are true coelomates in which the coelom is lined both on the inside of the body wall as well as around the gut by mesoderm. Animals with a true coelom also have mesenteries, which suspend the body organs within the coelom.Animals higher to round worms such as annelids, arthropods, mollusks, echinoderms and chordates fall in this category. Tue coelomates are of the following types.(i) Schizocoelomates are true coelomates in which the body cavity originates by splitting of mesodermal tissue at the time of gastrulation.. Pseudocoelomates, in which a body cavity exists in addition to alimentary canal, but it is lined by mesoderm only on the outer body wall side and not around the gut. Examples are round worms (Nemathelminthes) and some minor phyla grouped under Aschelminthes. 3. Coelomates or Eucoelmates.
Skeletal System Echinoderms are a phylum of marine animals. The adults are recognized easily by their radial symmetry, and include such well known animals as sea stars, sea urchins, sand dollars, and sea cucumbers. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssa zone.