Define haemocoel and true coelom.

Haemocoel:
Haemocoel (or hemocoel) refers to a body cavity found in certain invertebrates, particularly arthropods and mollusks. It is a spacious cavity that surrounds the organs and is filled with hemolymph, which is the circulatory fluid of these animals. In organisms with a haemocoel, the organs are bathed directly in hemolymph rather than being surrounded by a fluid-filled true coelom.

True Coelom:
A true coelom is a body cavity that is completely lined with mesoderm, one of the three primary germ layers in the embryos of animals. This cavity develops within the mesoderm and is distinct from the gut (digestive tract). The presence of a true coelom allows for more complex body plans and organization of internal organs. Animals with a true coelom are termed coelomates and include vertebrates and many invertebrates such as annelids (e.g., earthworms), mollusks (e.g., snails), and arthropods (e.g., insects).

In summary, the main difference between haemocoel and true coelom lies in their structure and development: haemocoel lacks a mesodermal lining and is filled with hemolymph, while true coelom is lined with mesoderm and contains organs in a fluid-filled cavity.