As protostome phyla evolve, the coelom may change in structure and function. For example, in some phyla like annelids, the coelom functions as a hydrostatic skeleton. In arthropods, the coelom may be reduced or lost altogether.
The lining of the coelom is called the mesothelium. It is a thin layer of cells that surrounds the coelomic cavity and helps to support and protect the organs within it.
The splanchnocoel is not a true coelom; rather, it is a type of body cavity found in certain organisms, particularly in some invertebrates like arthropods. It is derived from the embryonic mesoderm but is not entirely lined with mesodermal tissue, which is a defining characteristic of a true coelom. In contrast, a true coelom is a fully enclosed body cavity that is completely lined by mesoderm. Therefore, while the splanchnocoel serves as a body cavity, it does not meet the criteria to be classified as a true coelom.
Insects have a body plan characterized by a hemocoel, which is a type of body cavity that is not a true coelom. Instead of a coelom, insects possess a pseudocoelom-like space filled with hemolymph, which is a fluid that helps in nutrient transportation and waste removal. This hemocoel is part of the open circulatory system typical of arthropods. Hence, insects do not have a true coelom or a pseudocoelom in the traditional sense.
C. hemocoel
The sponges are Acoelomates. That is they don't have coelom or body cavity.
They express the characteristics inherent to arthropods. All insects are arthropods. Not all arthropods are insects.
Annelids, mollusks, and arthropods are classified as invertebrates, which are animals lacking a backbone. They belong to the larger group called Coelomates, characterized by having a true coelom, a fluid-filled body cavity. Annelids are segmented worms, mollusks include diverse organisms like snails and octopuses, and arthropods encompass insects, arachnids, and crustaceans, all distinguished by their body structures and developmental processes.
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The tissue derived from mesoderm that lines the coelom of annelids and forms walls is called mesothelium.
They express the characteristics inherent to arthropods. All insects are arthropods. Not all arthropods are insects.
The shiny lining of the coelom in a perch fish is called the peritoneum. It is a thin membrane that lines the body cavity and provides protection for the internal organs.