Question

Which animals do not have a Circulatory system?

Answer 1

The circulatory system, also known as the cardiovascular system or the vascular system, is an organ system which allows blood to circulate and transport nutrients $O_2$, $C{O_2}$, hormones, and blood cells to and from the cells in the body to provide nourishment and help in fighting diseases, stabilize temperature and pH, and maintain homeostasis.

Complete answer:
The circulatory system is the primary method used to transport nutrients and gases through the body in most of the multicellular organisms. Whereas in organisms which are only a few cell layers thick, in such animals simple diffusion takes place which allows some water, nutrient, waste, and gas exchange. However, bulk flow is the only method by which the essential components of the body travel in more complex organisms.

The circulatory system is generally composed of a network of cylindrical vessels: the arteries, veins, and capillaries that emanate from a pump i.e. the heart. In all vertebrate organisms, as well as some invertebrates, the cardiovascular system is a closed-loop system, in which the blood is traveled through blood vessels and circulates unidirectionally from the heart around the circulatory vessels and then returns again to the heart.

The simplest animals, for example the sponges (Porifera) and rotifers (Rotifera), do not need a circulatory system, so they do not have circulatory system because diffusion is sufficient to allow adequate exchange of water, nutrients, and waste, as well as dissolved gases.

Some organisms that are more complex but still only have two layers of cells in their body plan such organisms also use diffusion through their epidermis and internally through the gastrovascular compartment for example jellies (Cnidaria) and comb jellies (Ctenophora). In such organisms both their internal and external tissues are bathed in an aqueous environment and the fluids are exchanged by diffusion on both sides. In jellyfish fluids are assisted by the pulsing of the jellyfish body.

Note:
In more complex organisms diffusion is not sufficient for cycling nutrients, gases or waste products through the body that is why more complex circulatory systems evolved. In an open circulatory system blood is pumped out of the heart into a cavity and all the organs bathed into that cavity, whereas in a closed circulatory system blood is pumped into blood vessels and through these vessels blood travels to the different organs. Closed circulatory system is a characteristic feature of vertebrates.