Complex sentence: Each "individual" with its sail is really a hydroid colony, with many polyps that feed on ocean plankton and are connected by a canal system that enables the colony to share whatever food is ingested by individual polyps.
Split into simpler sentences: Each "individual" with its sail is really a hydroid colony, with many polyps that feed on ocean plankton. These are connected by a canal system that enables the colony to share whatever food is ingested by individual polyps.