Question

Define haemocoel and true coelom.

Answer 1

Hint: A coelom is the characteristic feature of metazoans. The true coelom is a body cavity formed at the embryonic stage from the three germinal layers. The coelom is lined by mesoderm epithelial cells. The presence and absence of coelom become a characteristic feature to differentiate animals.

Complete answer: Haemocoel arises in the early development, but later it is almost completely obliterated by a greatly enlarged blastocoel. The blastocoel consists of sinuses that are filled with blood. Blood flows within this cavity and then bathes the internal organs of the body. Gonads are never differentiated from the haemocoel wall, therefore, the coelom is confined to the cavity of excretory organs and ducts only. On the other hand, the true coelom originally splits within the embryonic mesoderm. The cavity of the coelom is bounded by a lining of mesodermal cells known as peritoneum. The coelom is a fluid-filled main body cavity which is present between the intestinal canal and on the body wall of echinoderms and chordates. It is specifically linked with mesodermal epithelium. On the other hand, acoelomates or haemocoel lack a true coelom. Haemocoel is a type of primary body cavity filled with circulatory fluids. Blood circulates through the haemocoel. Haemocoel is a reduced form of a coelom.

Additional information: The main difference between coelom and haemocoel is that a coelom is the principal body cavity of annelids, echinoderms and chordates that had originated from the mesothelium, while haemocoel is especially the body cavity of animals belonging to arthropods and molluscs, which is a reduced form of a coelom. Most multicellular animals have body cavities filled with fluids that surround organs. The body cavity inside the body is commonly called coelom. However, not all organisms possess a coelom.

Note: Though haemocoel and true coelom have similarities as both are filled with fluids and act as a cushion to protect internal organs. Both the cavities act as a hydrostatic skeleton. Both of them allow internal organs to grow, develop and change over time.