Cellular Defense Systems of the Echinodermata

Abstract

The echinoderms are of particular interest in phylogenetic studies because of their relatedness to the vertebrate subphylum. During the early Precambrian period, the animal kingdom is believed to have diverged into two major groups, known as the deuterostomes and the protostomes. Both the echinoderms and the vertebrates are deuterostomes, while all other invertebrates are classified as protostomes. The two groups are distinguished on the basis of major differences in early development, which may suggest that they were derived from different ancestors. “Protostome” is Greek for “mouth first” and in this group, the mouth is derived from the embryonic blastopore. The blastopore gives rise to the anus in deuterostomes (“mouth later”), while the mouth develops as a separate invagination. The pattern of cell cleavage also varies in the two groups. Cells are oriented in a spiral pattern in protostomes, whereas the blastomeres are positioned directly upon one another in the deuterostomes, resulting in a radial cleavage pattern. Early separation of the blastomeres in deuterostomes gives rise to complete individuals; this type of development is known as indeterminate cleavage. It does not occur in protostomes, where removal of blastomeres results in deformed embryos. Cleavage is said to be determinate in the latter case. During later development, the two groups also differ in formation of mesoderm and the coelomic cavity.