Phylum Placozoa Characteristics

• Metazoans are eukaryotic, multicellular organisms that undergo embryogenesis through tissue layers
• The phyla Ctenophora and Placozoa, along with two other phyla, diverge evolutionarily. They lack bilateral symmetry and a true mesoderm
• Phylogenetic analyses suggest that Porifera and Ctenophora are among the earliest branches in the animal kingdom

Phylum Characteristics - Placozoa

• Tiny (2-3 mm), flattened metazoans composed of upper and lower layers of ciliated cells

• The lower cell layer can temporarily invaginate, presumably for feeding

• Placozoans have six somatic cell types

• They possess cells with connections between them

• Placozoans have unique, bright spheres, possibly defensive structures, in the upper cell layer

• Placozoans lack a structural nervous system, muscles, or digestive system

• They reproduce asexually by fission and budding

• Eggs have been observed within the mesenchyma, but their origin is unknown

• Trichoplax adhaerens was discovered by F. E. Schulze in 1883 in a marine aquarium in Austria, and has been reported in tropical and subtropical seas

• Examples include Hoilungia hongkongensis, which was reported in 2018, and Poliplacotoma mediterranea, which was reported in 2019

Phylum Characteristics - Ctenophora

• An organism with two embryonic tissue layers (or even three, since they possess a mesenchyme) separated by a cellular mesenchyme
• They have biradial symmetry, with the main axis being the oral-aboral axis
• Ctenophora have adhesive cells called colloblasts
• These organisms possess a gastrovascular opening, a mouth-like invagination (stomodeum) and a digestive system ending in two small anal pores
• No developed respiratory, excretory, or circulatory systems are seen
• Musculature is always formed by true mesenchymal cells
• Ctenophora are monomorphic, without alternating generations or a sessile stage
• During some part of their life cycle, they have eight rows of ciliated plates (combs). The beating of the swimming palettes is controlled by a sensory apical organ
• Some adults possess a pair of long tentacles, which can be retracted in some and have a characteristic larvae
• Most are hermaphroditic

Order Cydippida

• Pelagic organisms that have well-developed rows of crests, long and retractable tentacles and a globular or ovoid body that is occasionally flattened
• Examples include Aulacococtena, Bathyctena, Callianira, Dryodora, Euplokamis, Haeckelia, Hormiphora, Lampea and Mertensia

Order Platyctenida

• Can be either planktonic or bentonic organisms
• They have only a few or no rows of crests
• Most species are very flat, with an everted pharynx that acts as a creeping sole
• They commonly have oral adhesive organs and sheaths and bifurcated tentacles
• Have anal pores, and are often comensals
• Internal fertilization and embryo development is more comon than in other ctenophores

Order Cestida

• Cestida are pelagic organisms
• Their bodies are extremely compressed in the tentacular plane and very elongated
• Can be up to 1 meter long in some species

Order Lobata

• Lobata are pelagic marine animals
• The lateral body is compressed with a pair of oral lobes and 4 auricles
• One example if the Black Sea comb jelly Mnemiopsis leidyi

Order Thalassocalycida

• Bodies are extremely fragile, expanded into a medusa-like bell, can reach 15 cm on the tentacular axis, slightly compressed during stomodeal plane
• Auricles and tentacular sheaths are absent; the tentacle appears close to the oral side with lateral filaments

Order Beroida

• Pelagic species with well-developed crest rows and long, retractable tentacles in sheaths
• Body shape globular or ovoid and occasionally flattened in the stomodeal plane (~sagittal)
• Some examples include Aulacococtena, Bathyctena, Callianira, Dryodora, Euplokamis, Haeckelia, Hormiphora, Lampea, and *Mertensia