Unicellular and Multicellular Animals 
sea-anemone and the Medusa, has probably been in many respects 
parallel to that which started from some primitive extinct form, 
and led up to the Ctenophora. Therefore the study of the group 
has thrown much light on the earlier history of the animal world. 
Regarding the age of the group, it may be mentioned that fossil 
corals, etc., are found, along with Crustaceans and Molluscs, in 
the earliest known fossil- bearing beds, belonging to the Cambrian 
age. 
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