PhD thesis - Biologisk Institut - Københavns Universitet

35 General discussionWhile Åkesson (1958) described the embryology and neurogenesis inPhascolion strombi, he also reported a growth zone that is situated anterior to theretractor roots. In addition, he showed that with the exception of P. strombi thegrowth zone in sipunculan larvae remains in this anterior position for only a part oftheir life cycle. However, in P. strombi the introvert retractors are attached at the veryposterior end of the body. Moreover, during the juvenile stages the left nephridiumdegenerates and the ventral and dorsal retractors fuse, with the latter becoming moreprominent, while the ventral retractor often degenerates (Åkesson 1958). P. strombihas a short larval development as well as fusion and degeneration processes in thefirst juvenile stages. Accordingly, similar to Åkesson’s description, a putative growthzone in T. pyroides and T. nigra is only present in their pelagosphera larvae prior tometatmorphosis (chapter IV). However, the question remains whether and how longsuch a putative growth zone may exist during ontogeny of P. strombi orPhascolosoma agassizii, since both display different developmental pathways than T.pyroides and T. nigra.Recently, ontogenetic studies showed that segmentation in annelids is morevariable than previously thought (Seaver et al. 2005, Brinkmann and Wanninger 2008,2010b). For instance, the segments in the polychaetes Capitella and Hydroides areformed from a ventrolateral region of the body, while a posterior growth zone is onlyevident in post-metamorphic stages (Seaver et al. 2005). This corroborates the data onanother polychaete, Sabellaria alveolata, where no larval posterior growth zoneappears (Brinkmann and Wanninger 2010b). Moreover, this variability of segmentformation is also mirrored in the nervous and muscle system formation in Sabellariaalveolata, where certain parts develop synchronously and others subsequently in ananterior to posterior manner (Brinkmann and Wanninger 2008, 2010b). Clearly, moreontogenetic studies, preferebly on supposedly basal polychaetes such as Dinophilidae,Oweniidae, and Protodrilida, are needed to shed more light on segment formation inAnnelida (including Sipuncula).Myogenesis in sipunculans and annelidsIn all sipunculans investigated so far, myogenesis follows a similar pattern. Fourintrovert retractor muscles begin to develop together with a considerable number ofcircular muscles (Wanninger et al. 2005a, Schulze and Rice 2009; chapter IV herein).The planktotrophic species Phascolosoma agassizii and Nephasoma pellucidum