This study presents the results of the research on natural history of Ostracoda (Crustacea: Arthropoda). Until now, most of the extant ostracods have been regarded as having a non-segmented body plan; however, the vestiges of trunk segments are generally found in many lineages and this character makes it possible to discuss the evolution of ostracod body plan. The distributional pattern of the pore systems on the carapace is a species-specific character in ostracods and the identity of this pattern among species through the ontogenetic process strongly suggests a phylogenetic relationship among them. The hinge structure has been traditionally regarded as an important character for higher classification. However, a new aspect of ostracod phylogeny has emerged with the introduction of the concept of heterochrony, leading to the novel discovery of an evolutionary trend in ostracods. A series of field surveys revealed that the primary adaptation of ostracods from sediment surface to interstitial environments occurred on coarse sand deposits in eulittoral zones, and then the ostracods secondarily adapted to the interstice of finer deposits in littoral zones, with a reduction in body size.