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Types of Circulatory Systems in Animals
Animals without circulatory systems
Simple animals consisting of a single cell layer, such as the (a) sponge, or only a few cell layers, such as the (b) jellyfish, do not have a circulatory system. Instead, gases, nutrients, and wastes are exchanged by diffusion.
Source
Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:
"OpenStax College, Overview of the Circulatory System. October 17, 2013."
http://cnx.org/content/m44801/latest/Figure_40_01_02ab.jpg
OpenStax CNX
CC BY 3.0.