腹足类个体发育中壳质结构的变化

腹足类个体发育中壳质结构的重要特点是壳质层的微观变化，包括原有壳质层的增厚、增生与上覆超微结构相同的壳质层、增生新的超微结构层、壳质层的相互消长与显微结构的演变。壳质层的微观变化决定了壳饰形成的４种类型：增厚型、刺顶型、刺穿型、叠覆型。叠覆的交错片体具加强贝壳抗破裂功能；交错片体排列方式的变化或新的显微结构层的出现均具分类意义。交错针状结构源于纤柱结构，纤柱结构又由简单柱状结构演变而来。

MICROVARIETIES OF GASTROPOD SHELL STRUCTURE DURING ONTOGENY

In general,the gastropod shell is gradually thickened along with shell growth.In addition,it is obviously thickened in ornamentation,aperture and proximity of the suture.Upon careful observation under scann ng electron microscope, it can be found that the shell thicknes is obviously controlled by the varieties of shell layers.The shell thickening is generally shown by the following varieties of shell layers:(1)Shell-thickening of original shell layer. For example,the prismatic layer is thickened by prolonging the prism. (2) Accretion of a shell layer with the same ultrastructure and arrangement as the above shell layer under the original shell layer. For example,a new prismatic layer occurs under the original prismatic layer (PI. I,fig. 8).(3) Accretion of a shell layer with the same ultrastructure but different arrangement as the above shell layer. That is,a concentric crossed lamellar layer appears under the radial crossed lamellar layer (PI. I,fig. 6). (4) Accretion of a shell layer with new ultrastructure. For example,a new fibrous prismatic layer suddenly occurs under a prismatic layer (PI. I,fig. 3). (5) Mutual growth and decline of adjacent shell layers (PI. n,fig.3) and changes in ultrastructure (PI. E,fig. 4).