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Where, as among the compound Hydrozoa, there is a union of many such primitive groups as form Hydræ; or where, as in a Medusa, one of these groups has become of great size; there exist rude channels running throughout the substance of the body: not, however, channels for the conveyance of prepared nutriment, but mere prolongations of the digestive cavity, through which the crude chyle-aqueous fluid reaches the remoter parts, and is moved backwards and forwards by the creature's contractions.
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