Invertebrate digestive systems
(a) A gastrovascular cavity has a single opening through which food is ingested and waste is excreted, as shown in this hydra and in this jellyfish medusa. (b) An alimentary canal has two openings: a mouth for ingesting food and an anus for eliminating waste, as shown in this nematode.
Source
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"OpenStax College, Digestive Systems. October 17, 2013."
http://cnx.org/content/m44736/latest/Figure_34_01_04ab.jpg
OpenStax CNX
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