Animal Phyla: A Comprehensive Look at Eumetazoa, Diblastica, and Triblastica
Animal Phyla: Eumetazoa, Diblastica, and Triblastica
Eumetazoa – Important Phylum:
Diblastica: Phylum Cnidaria
Cnidarians have simple body structures. They inject a toxic liquid using specialized cells. Morphologically, they are divided into two categories: polyps (sessile) and medusae (free-swimming). All cnidarians live in aquatic environments.
Triblastica:
Acoelomates: Phylum Platyhelminthes (Flatworms)
Platyhelminthes have flat bodies with tissues, organs, and some possess a simple nervous system. Some are free-living, while others are parasitic.
Pseudocoelomates: Phylum Nematoda (Roundworms)
Nematodes have long, cylindrical bodies with relatively simple tissues and organs. Some are free-living, while others are parasitic.
Coelomates:
Phylum Mollusca
Molluscs have soft bodies typically composed of a head (containing the mouth and sensory organs), a muscular foot for locomotion, and a visceral mass containing the internal organs. Many molluscs secrete a calcareous shell. Most molluscs are aquatic, but some are terrestrial.
Classes:
- Gastropoda: Possess a single shell or no shell (e.g., snails).
- Bivalvia: Have two hinged shells (e.g., clams).
- Cephalopoda: Shells may be internal, external, or absent (e.g., octopus).
Phylum Annelida (Segmented Worms)
Annelids exhibit a relatively complex organization. Their bodies are metameric, meaning they are divided into segments or metameres. This segmentation extends internally, with repeated organs in each segment. They inhabit aquatic and moist terrestrial environments, and some are parasitic.
Classes:
- Polychaeta: Possess numerous bristles (setae) on parapodia (e.g., nereis).
- Oligochaeta: Have few setae and lack parapodia (e.g., earthworms).
- Hirudinea: Lack setae and possess suckers for attachment (e.g., leeches).
Phylum Arthropoda
Arthropods are the largest and most successful animal group, colonizing diverse habitats. Their bodies are segmented, exhibiting metamerism, but unlike annelids, the segments are not all identical. The body is typically divided into three parts: the head, thorax, and abdomen. In some groups, the head and thorax are fused into a cephalothorax. Each segment bears jointed appendages specialized for different functions. Arthropods are covered by a rigid or flexible chitinous exoskeleton, which may be reinforced with calcium salts. The exoskeleton protects against predators and prevents water loss. To grow, arthropods undergo ecdysis, shedding their exoskeleton.
Subphyla:
- Chelicerata: Includes classes Merostomata and Arachnida.
- Mandibulata: Includes classes Myriapoda, Crustacea, and Insecta.
Phylum Echinodermata
Echinoderms possess an internal skeleton (endoskeleton) composed of calcareous plates, sometimes with spines. Their bodies are not segmented, and they lack a distinct head. Adults exhibit radial symmetry as a secondary adaptation, while larvae are bilaterally symmetrical. A unique water vascular system, consisting of fluid-filled canals, facilitates locomotion and food capture. Echinoderms are exclusively marine, with some being sessile or slow-moving.
Phylum Chordata
Chordates are characterized by the presence of a notochord (a flexible rod providing support) at some stage in their life cycle. In some chordates, the notochord is replaced by a vertebral column. Chordates possess a dorsal, hollow nerve cord located above the notochord. The anterior end of the nerve cord develops into the brain (encephalon) in vertebrates. Pharyngeal slits are present in the pharyngeal wall. In aquatic chordates, these slits develop into gills, while in terrestrial chordate embryos, they do not fully develop. Chordates also have a post-anal tail and a ventral heart.
Subphyla:
- Urochordata
- Cephalochordata
- Vertebrata