Nematoda
There are about one million nematoda species known to man. Nematoda species have successfully adapted to every possible environment. This includes salt water, fresh water, brackish water, tropics, arctic and so on. They are also known as roundworms. They are wormlike and are surrounded by an exterior layer of non-cellular cuticle. Nemotodas do not have a cardio-vascular or circulatory system. They transfer oxygen and necessary nutrients by the act of diffusion.
This video shows the anatomy of nematodas or roundworms. You can see that they have no heart.
This video shows the anatomy of nematodas or roundworms. You can see that they have no heart.