Cnidaria (also called coelenterate) are found exclusively in an aquatic environment (Dorit et al, 1991). The phylum Cnidaria contains nearly 9,000 species and is made up of four classes: Hydrozoa, Scyphozoa, Anthozoa and Cubozoa (Encyclopaedia Britannica 2009). Cnidaria are diploblastic – having two main cell layers, and are either radially or biradially symmetri-cal (Fautin and Romano 1997).Table 1. Common features of the phylum and its four classes (Dorit et al, 1991; Encyclo-paedia Britannica 2009; Fautin 2002; Fautin and Romano 1997; Grzimek (ed.) 1984; Wiki-pedia 2011)

Figure 1. Examples of five different Cnidarian species a. Physalia physalis also known as “Portuguese man-of-war” - a colony of pol-yps – Class Hydrozoa (King 2010) b. Acropora cervicornis also known as Staghorn coral – Class Anthozoa (Bruckner 2002) c. Chrysaora colorata also known as the Mauve Stinger and the Purple Striped Jellyfish – Class Scyphozoa (Watanabe, 2010) d. Anemo-nia viridis also known as snakelocks anemone – Class Anthozoa e. Carybdea sivickisi also known as a “box jellyfish” – Class Cubo-zoa (UCMP 2009)

a.

b.

c.

d.e.

Physalia physalis versus Anemonia viridisTable 3. Comparison of Physalia physalis and Anemonia viridis (Eaker 2003; Fautin 2010; Guzik 2007; Hoover 2007; King 2010)

Table 2. Complete scientific name for Physalia physalis and Anemonia viridis (Bruckner 2002; Guzik 2007)

Figure 2. (Above) Labelled diagram of Anemonia viridis (Sabourault et al, 2009)Figure 3. (Right) Labelled photograph of Physalia physalis (Anon b, 2009)