The present study contributes to many major topics in the developmental biology of fishes: the development of the early respiratory organs, the differentiation of red muscle layer and its specific function, the deveiopment of the visual system, the time of the first feeding, the development of mechanosensory organs (free neuromasts) and finally the development of scales,A provocative question on these topics could be asked: how growing structures adapt themselves to the constantly changing demands of function and behaviour? Morphological, histological and scanning electron microscopic investigations were combined to study the ontogenetic changes in some selected structures of the developing embryos and larvae of the big head carp, Aristichthys nobilis,It was found that the newly hatched larvae have four gill arches. After three days of hatching, small protuberances (filaments) were found on these arches during which such gills were not yet functional. Larvae of 4-5 days old possess four gill arches with series of relatively short filaments, each possesses numerous secondary lamellae (the true respiratory surface) on both sides.The muscle regions in the pre-hatched embryo consist of scattered undifferentiated myoblasts. The body of the recently hatched larva was surrounded by a single layer of muscle fibers (the gills were still in a rudimentary state). The larvae respire almost exclusively through the superficial layer of red muscle fibers. This layer is to great extent short after hatching but diminishes gradually by contracting towards the lateral region of the body until it merges with (or gives rise to ) the proper red muscle fibers of the adult. The present study suggests that there is a close relationship between the rate of differentiation of gill structures and the rate at which the larval red muscle layer disappears. It seems quite clear the that red layer of muscle fibers and the secondary lamellae of gills serve as organs of gas exchange at different stagesof development. This is expected if one organ is to replace the other without causing a break in respiratory function.In newly-hatched larvae, free neurornasts were already recognized in both the head and trunk regions. During growth, the number of free neurornasts increased. In the head, free neurornasts were located around the eye and nose, their direction of maximum sertsitivity.* Distinct changes in free-neuromasts occurred during the formation of the canal organ. The free neurornasts were become functional as a mechanoreceptors when they well developed cupulae and can detect the origin of mechanical stimuli at this very early stage.The visual system is poorly developed and free of pigment at hatching, and eyes were probably non-functional at this time. The development of eye pigmentation should provide a first feeding larvae with sight.As the larvae develop, the number of the surface neurornasts gradually decreased and finally disappeared. At the same time, the canal organ was formed and the scales has advanced. The lateral-line scales extend posteriorly nearly the tail and anteriorly nearly to the pectoral girdle. Our data show clearly that the disappearance of the free neurornasts was correlated with the appearance of both retinal pigments and body scales.It seems quite clear that right the sequence of the development of the structures necessary for vital functions appears to be'well adapted to the demands.