979. THE SHELL STRUCTURE OF THE CARAPACE IN XESTOLEBERIS HANAII ISHIZAKI (CRUSTACEA, OSTRACODA)

Abstract

The development of carapace shell structure during ontogeny of Xestoleberis hanaii Ishizaki, 1968 was investigated with the aid of a scanning electron microscope (SEM). Photographs of broken sections of carapaces ranging from the second instar (the adult-7 instar) to adult reveal that the carapace is composed of four distinct layers : 1) an organic fibrous layer that is characterized by a network of organic fibers with few calcite grains ; 2) a foliated layer formed by an aggregation of plate-like grains ; 3) a granular layer consisting of an aggregation of granular grains ; and 4) a prismatic layer constructed of an aggregation of columnar grains extending perpendicularly to the valve surface. Three of the layers, i. e., all but the organic fibrous layer, are distinguished by the shape and arrangement of calcite grains. The calcified procuticle of adult specimens is composed of an outer organic fibrous, a middle granular, and an inner prismatic layer. That of juvenile specimens in the second to fourth instars consists of only the foliated layer. That of juvenile specimens in the fifth and sixth instars develops outer granular and inner prismatic layers. The seventh and eighth instar juveniles possess a procuticle of the same three layers as in the adult. During growth, the shell structure of this species thus undergoes a change in its constructional design between molt stages, particularly from fourth instar to fifth instar and from sixth instar to seventh instar.