They can live inside the gastrointestinal tract or, the case of roundworms, in the lymphatic system.
It is because of your bodies little-known secondary circulation system called the lymphatic system.
lymphatic system n.
The interconnected system of spaces and vessels between body tissues and organs by which lymph circulates throughout the body.
|lymphatic system |
A network of vessels, tissues, and organs in vertebrate animals that helps the body regulate fluid balance and fight infection. The vessels of the lymphatic system drain excess fluid, called lymph, from the tissues and return it to the circulating blood. Lymphocytes circulate throughout the lymphatic system.
The network of small vessels and tissue spaces that move lymph throughout the body. The lymphatic system has several functions, including filtering out harmful bacteria; manufacturing white blood cells (white blood cells are produced by the lymph nodes); distributing nutrients to the cells of the body; helping to maintain the body's fluid balance by draining off excess fluids so that tissues do not swell; and assisting in the digestion of fats.