Fig. 11 in Morphogenetic gradients in graptolites and bryozoans

Fig. 11. Astogeny of bryozoan colonies. A. Diagram based on Escharoides Milne Edwards and showing morphological variation in relatively simple bryozoan colony. All zooids have originated from the primary oozooid (A, ancestrula) and display a gradient in size and shape of zooecia until first repetitive zooids appear (3). This proximal series of zooids compose together the primary zone of astogenetic change. Further development leads to a series of zooids showing the same size and shape and making the primary zone of astogenetic repetition. A series of zooids near the growing edge (5–8) display a growth gradient, all of them will reach eventually morphology displayed by 4. B. Astogeny in a cheilostomate bryozoan Poricellaria d'Orbigny showing besides primary zone of astogenetic change and repetition, also cyclically repeating subsequent zones of change (S1C, S2C) as well as of repetition (S1R, S2R). While primary zone of astogenetic change begins with the ancestrula (A) subsequent zones of astogenetic change start with diminutive zooids which are not wholly comparable with ancestrula. In the given zone of astogenetic repetition zooids are alike and their morphology repeats this of the last generation of the preceding zone of astogenetic change. Successive zones of change occur over fewer generations, their zooecia become longer, more asymmetrical, may also change the budding pattern. Modified from Boardman et al. (1969).