Cyclostomatida - Differentiation

Differentiation

A colony is founded by a larva which settles and metamorphoses into a zooid, the ancestrula. In Cyclostomata it may be of two types: holoancestrula and artroancenstrula (in crisiids) The ancestrula gives rise to a second generation of zooids, which in turn gives rise to a third, and so on, in a process called astogeny. Generational variation among zooids is called astogenetic differentiation. In all species, there is a primary zone of differentiation, which is limited to the first few generations and followed by a much longer zone of repetition of nearly identical zooids. In some species, however, there is a secondary differentiated zone, which can take various forms. In species in which the colony branches, new branches normally arise by division of a distal growing tip of an existing branch. An adventitious branch, in contrast, is one that arises from the side of an existing branch, beginning with a short series of differentiated zooids, a secondary zone. Subsequent generations of zooids along the branch then typically return to the normal colony budding pattern.