Patterns Of Development

The distinct patterns of cleavage and formation of body-plan features found in different animal phyla are clues to the phylogenetic history of the organisms.

Types of Body Cavities

Animals, such as flatworms, that do not have a body cavity are called acoelomates (uh-SEE-luh-MAYTS). The interior of the animal is solid, as shown in Figure 32-14a. The endodermic gut, shown in yellow, and the outer covering of the animal, shown in blue, are connected by the solid tissue of the mesoderm.

However, most animal phyla have body cavities that separate their digestive tract from the outer body wall. Within this group, there are differences in how the body cavity develops. In some phyla, including rotifers and roundworms, the mesoderm lines the interior of the coelom but does not surround the exterior of the endodermic gut. A cavity that is not completely lined by mesoderm is called a pseudocoelom (soo-doh-SEE-luhm), which means "false body cavity." Roundworms have pseudocoeloms such as the one shown in Figure 32-14b and are thus called pseudocoelomates. In pseudocoelomates, mesoderm lines the fluid-filled body cavity, and the endodermic gut is suspended in this fluid.

A cavity completely lined by mesoderm is called a coelom (SEE-luhm), as shown in Figure 32-14c. Animals that have coeloms are called coelomates (SEE-luh-MAYTS). In coelomates, mesoderm lines the body cavity and surrounds and supports the endodermic gut. The mesoderm also forms the tissues of attachment for the organs located in the coelom, such as the liver and the lungs. Mollusks, annelids, arthropods, chordates, and echinoderms are coelomates.

Cleavage and Blastopore Fate

Recall from Figure 32-3 that echinoderms and chordates share a branch of the phylogenetic diagram of animals; and mollusks, annelids, and arthropods share another branch. There are two distinct patterns of development in animals with a coelom. In the embryos of mollusks, arthropods, and annelids, the blastopore develops into a mouth, and a second opening forms at the other end of the archenteron, forming an anus. These organisms are called protostomes (PROHT-oh-STOHMZ), which means "first mouth."

Many protostomes undergo spiral cleavage, in which the cells divide in a spiral arrangement. In the embryos of echinoderms and chordates, the blastopore develops into an anus, and a second opening at the other end of the archenteron becomes the mouth. These organisms are called deuterostomes (DOOT-uhr-oh-STOHMZ), which means "second mouth." Most deuterostomes undergo radial cleavage, in which the cell divisions are parallel to or at right angles to the axis from one pole of the blastula to the other.

Endoderm I Mesoderm Ectoderm

Skin

I Tissue-filled region

Endoderm I Mesoderm Ectoderm

Skin

I Tissue-filled region

(a) ACOELOMATE

(a) ACOELOMATE

Skin

Muscle tissue

Muscle tissue

(b) PSEUDOCOELOMATE

Body cavity (pseudocoelom)

(b) PSEUDOCOELOMATE

Skin

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