A highly derived Genus within the Order Siphonophora, Physalia are nevertheless colonial organisms, as are all Siphonophores. The pneumatophore (float-like balloon) grows from the founding zooid (individual). This founding zooid also becomes the stolon (connective canal) from which subsequent zooids bud off in modified asexual reproduction. Unlike some forms of asexual reproduction, these zooids remain attached to the colony via the stolon. In times of distress they may break off from the colony as a defense mechanism known as autotomy.
Zooids other than the founder are highly polymorphic (morphologically and behaviorally specialized), and function much like human organs (stomach, gonads, skin, etc). Distinct zooids have evolved to digest, produce gametes, capture prey, and defend the colony. In other Siphonophores there may be over six distinct polymorphic zooids within a single colony. All zooids operate in both independent and coordinated behavior despite lacking a central nervous system. In fact Siphonophores have only been shown to possess a diffuse nerve net. Thus, Physalia are a collection of individuals, a colony.
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