MINELLI, A.; University of Padova: A morphologist’s perspectives on terminal growth and segmentation

In many lophotrochozoans and crustaceans, the main body axis is elongated by the proliferative activity of teloblasts. There are ecto- and mesoteloblasts, but no endoteloblasts: the elongation of the main body axis is led by the external �somatic animal�. From the point of view of polarized growth, the latter�s independence from the internal �visceral animal� is best shown by Phoronis: here, the agreement in axial arrangement between somatic and visceral animal found in the actinotrocha larva is abandoned when the somatic animal starts growing along a new axis. The distribution of polarized, subterminal growth along an animal�s ontogenetic schedule is subject to constraints. Continuity of function may oppose postembryonic shape changes, or contribute to restrict them to critical times such as moltings. Developmental constraints on body elongation are likely not to differ much between embryonic and post-embryonic life, as shown by the phylogenetic distribution of anamorphic vs. epimorphic developmental schedules in the arthropods. It is quite likely that the origin of segmentation in annelids, arthropods and vertebrates was related to efficiency in body elongation. With segmentation, cell proliferation and differentiation may go on in parallel (or with limited time delay) from as many growth points as there are groups of regularly spaced cells. The main consequence is not so much a more expedite elongation (the process is ultimately resource-limited, irrespective of the mechanism), as an important reduction of the disparity of metabolic conditions, gene expression patterns and �relative age� of different body districts, otherwise possibly troublesome within the limited space of the embryo. Subdivision of primary segments into secundary units also fits well into this adaptive interpretation of segmentation in relation to body elongation.