SpringerImages - 3D-reconstruction of the head skeleton and musculature of a 8.0 mm SL Ancistrus cf. triradiatus free-living embryo. The branchial basket is not shown. a Ventral view (right suspensorium and hyoid bar removed; only suspensorial and opercular musculature shown); b lateral view (jaw and maxillary barbel muscles removed). ch ceratohyale, hs hyosymplecticum, ih interhyale, m-add-apal musculus adductor arcus palatini, m-add-hm musculus adductor hyomandibulae, m-add-op musculus adductor operculi, m-dil-op musculus dilatator operculi, m-hh-add musculi hyohyoidei adductores, m-hh-inf musculus hyohyoideus inferior, m-lev-apal musculus levator arcus palatini, m-lev-op musculus levator operculi, m-stern musculus sternohyoideus, o-ch-a os ceratohyale anterior, o-den os dentale, o-op os operculare, o-pmx os praemaxillare, ot-cap otic capsule, r-br-III radius branchiostegalis III, t-dil-op tendon of musculus dilatator operculi

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Fig 2

3D-reconstruction of the head skeleton and musculature of a 8.0 mm SL Ancistrus cf. triradiatus free-living embryo. The branchial basket is not shown. a Ventral view (right suspensorium and hyoid bar removed; only suspensorial and opercular musculature shown); b lateral view (jaw and maxillary barbel muscles removed). ch ceratohyale, hs hyosymplecticum, ih interhyale, m-add-apal musculus adductor arcus palatini, m-add-hm musculus adductor hyomandibulae, m-add-op musculus adductor operculi, m-dil-op musculus dilatator operculi, m-hh-add musculi hyohyoidei adductores, m-hh-inf musculus hyohyoideus inferior, m-lev-apal musculus levator arcus palatini, m-lev-op musculus levator operculi, m-stern musculus sternohyoideus, o-ch-a os ceratohyale anterior, o-den os dentale, o-op os operculare, o-pmx os praemaxillare, ot-cap otic capsule, r-br-III radius branchiostegalis III, t-dil-op tendon of musculus dilatator operculi

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In this specimen the adductor arcus palatini has substantially expanded and has become a broad muscle plate originating from the trabecular bar and inserting on the base of the pterygoid process and the medial edge of the hyosymplectic of the cartilaginous suspensorium (Fig. 2 a; at this moment perichondral ossification is just starting to cover some parts of the chondrocranium, but not this region of the suspensorium).

The direction of the levator arcus palatini has slightly shifted: the muscle now runs rostroventrally instead of ventrally from the neurocranium–suspensorium articulation (Fig. 2 b).

The suspensorium has also elongated and its ventral part has shifted anteriorly (compare Figs. 1 , 2 b).

During ontogeny, several major transformations occur that influence the mode of respiration: the proceeding development of the cranial muscles and ligaments, and their insertions, the ossification of the skeletal elements, and the lengthening of the skull, including the change in shape and position of the suspensorium (compare Figs. 1 , 2 b).

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