Characteristics of Coelomates

Body Organization

• Coelomates: animals with a body cavity lined by mesoderm, e.g., annelids, molluscs, arthropods, echinoderms, hemichordates, and chordates.
• Pseudocoelomates: animals with a body cavity not lined by mesoderm, but instead, mesoderm is present as scattered pouches between ectoderm and endoderm, e.g., aschelminthes.
• Acoelomates: animals without a body cavity, e.g., platyhelminthes.

Segmentation

• Metameric segmentation: the body is externally and internally divided into segments with a serial repetition of at least some organs, e.g., earthworm.
• Metamerism: the phenomenon of serial repetition of organs.

Notochord

• Notochord: a mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals.
• Chordates: animals with a notochord, e.g., some animals from embryonic development.
• Non-chordates: animals without a notochord, e.g., porifera to echinoderms.

Body Symmetry

• Bilateral symmetry: the body can be divided into identical left and right halves in only one plane, e.g., annelids, arthropods.
• Radial symmetry: the body can be divided into identical parts around a central axis, e.g., coelenterates, ctenophores, and echinoderms.

Germinal Layers

• Diploblastic organization: animals with two embryonic layers, an external ectoderm and an internal endoderm, e.g., coelenterates.
• Triploblastic organization: animals with three germinal layers, an external ectoderm, an internal endoderm, and a mesoderm in between, e.g., platyhelminthes to chordates.

Coelom

• Coelom: a body cavity lined by mesoderm, present in some animals, e.g., annelids, molluscs, arthropods, echinoderms, hemichordates, and chordates.

Phylum Characteristics

• Porifera: cellular organization, absent symmetry, absent coelom, absent segmentation, and absent digestive, circulatory, and respiratory systems.
• Coelenterata (Cnidaria): tissue organization, radial symmetry, absent coelom, absent segmentation, and incomplete digestive system.
• Ctenophora: tissue organization, radial symmetry, absent coelom, absent segmentation, and incomplete digestive system.
• Platyhelminthes: organ organization, bilateral symmetry, absent coelom, absent segmentation, and incomplete digestive system.
• Aschelminthes: organ organization, bilateral symmetry, pseudocoelomate, absent segmentation, and complete digestive system.
• Annelida: organ organization, bilateral symmetry, coelomate, present segmentation, and complete digestive system.

Learn about the features of coelomates, including annelids, molluscs, and arthropods, and their body cavity structures.