Crayfish are invertebrates which means they don't have a backbone. They are also arthropods, which means that they have a protective exoskeleton on the outside of their body. Other arthropods include insects, spiders, and centipedes. Crayfish continually shed their exoskeleton and regrow a new one. During this transition is when they are most vulnerable to predators because they aren't protected by their outer shell.
Crayfish are found all over the world in many different freshwater environments, such as rivers, creeks, springs, and swamps. But this doesn't necessarily mean that you will be able to find them. They are somewhat cryptic because they dig out homes for themselves under rocks and blend in well with their environment. Crayfish will ardently defend this dwelling against intruders using their strong front claws
Crayfish can't grow unless the shell (comprising the carapace, or main body shell, tail shell, and leg shells) can be removed. And this is exactly what crayfish do. Periodically (quite often early in life) the crayfish slides out of its old, hard shell in a process called molting
Explanation:
It is about what structure do crayfish have to help them grow
The thymus is the only lymphoid organ that does not D. fight the antigens directly.
The thymus, bone marrow, spleen,lymph nodes, a network of lymphoid tissue along secretory surface (MALT), and the skin which performs the immune functions are known as lymphoid organs.
On the basis of its role, lymphoid organ are classified into primary and secondary lymphoid organ.
The lymphocytes from bone marrow are transformed into specialised T cells in the thymus. In this way thymus helps to improve the immune system
T cells produced in the thymus is travels through the blood stream to lymphnodes and other parts of the body to fight against the antigen.
