Phylum Porifera: Porifera, commonly known as sponges, are asymmetrical and lack cephalization. They do not possess a true gut or body cavity and have no segmentation. Porifera are characterized by the absence of true tissues, existing instead at the cellular level of organization. Their development pattern is minimal, lacking distinct embryonic layers or stages. Porifera reproduce both asexually through budding and sexually, often as hermaphrodites. A unique feature of sponges is their choanocytes (collar cells), which aid in filter feeding.

Phylum Cnidaria: Cnidarians exhibit radial symmetry and lack cephalization. They possess a sac-like gut and are acoelomate. Segmentation is absent in this phylum. Cnidarians are diploblastic, with two tissue layers: the epidermis and the gastrodermis, separated by a non-cellular mesoglea. Their development includes indirect patterns with a planula larva stage and radial cleavage. Reproduction occurs both asexually through budding and sexually. Cnidarians are well-known for their nematocysts, or stinging cells.

Phylum Platyhelminthes: Platyhelminthes, or flatworms, are bilaterally symmetrical and exhibit cephalization. They have a sac-like gut and are acoelomate, with no body cavity. Segmentation is absent. These organisms are triploblastic, possessing three germ layers. Their development follows the protostome pattern with spiral cleavage, and it can be either direct or indirect. Reproduction is predominantly sexual, with many species being hermaphroditic. Platyhelminthes are noted for their remarkable regeneration abilities.

Phylum Rotifera: Rotifers are bilaterally symmetrical and show cephalization. They possess a tubular gut and are pseudocoelomate, having a body cavity partially lined with mesoderm. Segmentation is not present. Rotifers are triploblastic and develop through a protostome pattern with spiral cleavage, typically undergoing direct development. They reproduce both sexually and asexually through parthenogenesis. A distinguishing feature of rotifers is their corona, a ciliated structure used for feeding and locomotion.

Phylum Annelida: Annelids are bilaterally symmetrical and exhibit cephalization. They have a tubular gut and are coelomate, with a true body cavity fully lined with mesoderm. Segmentation is a prominent feature. Annelids are triploblastic, and their development follows the protostome pattern with spiral cleavage, with both direct and indirect development seen. Reproduction is sexual, with some species being hermaphroditic. Annelids possess setae, or bristles, on their segments.

Phylum Mollusca: Molluscs are bilaterally symmetrical and exhibit cephalization. They possess a tubular gut and are coelomate. Segmentation is generally reduced. Molluscs are triploblastic and develop through the protostome pattern with spiral cleavage, with both direct and indirect development possible. Reproduction is sexual, and some species are hermaphroditic. Key characteristics of molluscs include the mantle, which secretes the shell, and the radula, a toothed feeding structure.

Class Gastropoda (within Mollusca): Gastropods, a class within the Mollusca phylum, are bilaterally symmetrical and exhibit cephalization. They have a tubular gut and are coelomate. Segmentation is absent. Gastropods are triploblastic and follow the protostome pattern of development, with both direct and indirect development. Reproduction is sexual, with many species being hermaphroditic. A notable feature of gastropods is torsion, a process that twists their body during development.

Class Bivalvia (within Mollusca): Bivalves, another class within Mollusca, are bilaterally symmetrical with reduced cephalization. They possess a tubular gut and are coelomate, with no segmentation. Bivalves are triploblastic and develop through the protostome pattern with spiral cleavage, typically undergoing indirect development. Reproduction is sexual. Bivalves are filter feeders and have a distinctive two-part shell.

Class Cephalopoda (within Mollusca): Cephalopods, a class within Mollusca, are bilaterally symmetrical and exhibit significant cephalization. They possess a tubular gut and are coelomate, with no segmentation. Cephalopods are triploblastic and develop through the protostome pattern with spiral cleavage, usually undergoing direct development. Reproduction is sexual. Cephalopods are known for their highly developed nervous system and complex behaviors.

Phylum Nematoda: Nematodes, or roundworms, are bilaterally symmetrical and exhibit cephalization. They possess a tubular gut and are pseudocoelomate. Segmentation is absent. Nematodes are triploblastic and follow the protostome pattern of development, exhibiting eutely, where individuals have a fixed number of cells. Development is direct. Reproduction is sexual, with some species being hermaphroditic. Nematodes are characterized by their cuticle and longitudinal muscles.

Phylum Arthropoda: Arthropods are bilaterally symmetrical and exhibit cephalization. They possess a tubular gut and are coelomate. Segmentation is a key feature. Arthropods are triploblastic and develop through the protostome pattern with spiral cleavage, with both direct and indirect development observed. Reproduction is sexual. Arthropods are distinguished by their exoskeleton and jointed appendages.

Class Polyplacophora (within Mollusca): Polyplacophorans, a class within Mollusca, are bilaterally symmetrical with reduced cephalization. They have a tubular gut and are coelomate, exhibiting segmentation. Polyplacophorans are triploblastic and follow the protostome pattern with spiral cleavage, typically undergoing direct development. Reproduction is sexual. A distinctive feature of polyplacophorans is their eight-plated shell.