IV. Classification of phyla
A. Asymmetric animals without germ layers.
1. Phylum PORIFERA (sponges) –
The sponges represent a dead-end phylum (that is they did not give rise to any other existing phyla) are benthic, sessile filter feeders with variable body sizes and complex water canals throughout their bodies. Sponges had very few competitors in early seas (see micro-feeding above), but had the need for defenses against predators, using both chemical and physical (sharp spicules) defenses. The commercial sponge trade has been replaced by the invention of synthetic sponges.
B. The radiates – diploblastic animals with radial symmetry.
1. Phylum: CNIDARIA (animals containing cnidocyte cells – e.g. coral, anemones, and jellyfish) –
These animals evolved when macroscopic protists were abundant, well after sponges evolved. These animals were sessile or slow-moving hunters that used attached paralyzing harpoons (nematocysts in cnidocyte cells) to capture prey or for defense. Sessile forms (e.g., coral) supplement energy with a mutualistic algae. Most cnidarians have a multipurpose, blind, gastrovascular cavity, extending to most of the body tissues for nutrient delivery.
C. Lophotrochozoan protostomes – Triploblastic animals with bilateral symmetry, a blastopore becoming a mouth, and showing growth by incremental additions to the body (without having to shed to grow).
1. Phylum: PLATYHELMINTHES (flatworms)
This phylum includes free-living flatworms and parasitic flukes and tapeworms. The most primitive groups include bilateral symmetry, cephalization, and typical tube-within-a-tube design. The flat body maximizes surface area for diffusion of gasses (they have no respiratory system). This group represents the first bottom-dwelling, flat, slow-moving scavengers, also including well-developed chemoreceptors for localizing food.
2. Phylum: ANNELIDA (segmented worms)
This group likely evolved from a free-living flatworm and includes earthworms, predatory marine worms, and leeches. Marine worms were the first annelids (and majority of existing annelids today). Later annelids radiated into freshwater and onto land. Annelids have a hydrostatic (liquid-inflated) skeleton and gripping setae, both of which assist in burrowing through substrate. Annelids have repetition of body parts along the body axis (segmentation) allowing independence of body parts. Earthworms are important in cycling nutrients in ecosystems.
3. Phylum: MOLLUSCA (chitons, snails, clams, squid, octopus and others)
The mollusks are the second largest phylum (first is Arthropoda) and extremely diverse, and are likely to have evolved from an annelid ancestor. Adaptive radiation is due to several innovations, including a mantle, protective shell, muscular foot, and a scraping radula, all allowing the early mollusks to feed on suspended and attached algae near shore. Only snails moved onto land. Many of the species are used for food. Some, like the Zebra Mussel, are problematic.
D. Ecdysozoan protostomes – Triploblastic animals with bilateral symmetry, a blastopore becoming a mouth, and showing growth via repeated shedding of the outer body exoskeleton (ecdysis).
1. Phylum: NEMATODA (nematodes or roundworms)
Members of this phylum occur in most habitats, including other organisms, and likely evolved from a flatworm ancestor. The number of individual nematodes is much greater than the number of all other animals combined. Nematodes are tolerant of a variety of extreme conditions, including drought (>39 years), freezing and boiling, O2 depletion, and low pH. Primary among their adaptations is the evolution of a cuticle (protective, dehydration-resistant body covering that is shed to permit growth. Nematodes play an active role in nutrient cycling. They are the most abundant multicellular organism that feeds on bacteria and fungi in decaying plants and animals. Nematodes are also a concern to animals, causing diseases like Trichinosis and Elephantiasis.
2. Phylum: ARTHROPODA (insects, crustaceans (crabs, lobster, shrimp, etc.), and spiders (mites, ticks, scorpions, etc.))
The arthropods dominate our landscape in abundance and numbers of species (80% of all known animal species – 70%of all animal species are beetles). Arthropods evolved from an annelid ancestor (arthropids have segmentally arranged appendages/bodies). The chitinous external skeleton (exoskeleton) was the innovation that met all the criteria for moving out of the water and onto land. The mollusks had heavy shells. The marine worms were segmented for flexibility, but movement with parapodia didn’t allow for speed on land. The arthropod’s exoskeleton has strength, is economical, and allows for speed of movement. Later arthropods evolved a tracheae system (small breathing tubes to the inner body parts) and a compound eye (for spotting food from a distance, rather than just bumping into it.
E. Deuterostome Phyla – Deuterostomata = “second mouth”; the mouth is the second opening to develop during embryonic growth, the anus being the first. In the Protostomes (= “first mouth”), the mouth is the first opening to develop, the anus being the second. Early embryological differences like these reveal the early divergence of two fundamentally different evolutionary lines of animal phyla, the protostomes and deuterostomes. Animals in the deuterostome phyla have bilateral symmetry, triploblastic germ layers, and eucoelomate body cavities.
1. Phylum: ECHINODERMATA (starfish (sea stars), sea urchins, and sea cucumbers)
Echinoderms represent the closest non-chordate relative to the chordates, based on genetic information and larval type. However, a common ancestor between echinoderms and the chordates is unknown. Overall, echinoderms are slow-moving, omni-directional, heterotrophs, with spines for protection. They have arms with gripping suction cups for locomotion and predation.
2. Phylum: CHORDATA (all with a notochord or more)
a. This phylum is comprised of progressively more mobile, more rigid-bodied heterotrophs. In addition to an increase in cephalization, there are 4 key chordate characteristics.
i. Notochord – fibrous support rod along the back appearing at least during early development (replaced by a vertebral column in the adult in later evolved chordates.
ii. Pharyngeal gill slits – initially used in filtering food suspended in water, but later just for respiration.
iii. Dorsal hollow nerve cord – a single dorsal nerve for rapid sensory processing.
iv. Post-anal tail – a portion of a tail extends posteriorly past the anus (effective for locomotion in early evolution).
b. There are 3 chordate subphyla: Urochordata, Cephalochordata, and Vertebrata. Within the Vertebrata there are 6 major classes:
i. Chondrichthyes – cartilaginous fishes (sharks and rays) – first chordates with jaws.
ii. Osteichthyes – bony fishes – includes all feeding modes (mostly carnivorous, but also algivores, suspension feeders, and scavengers.
iii. Amphibia – frogs, toads, and salamanders – first chordates to invade land, but have to stay close to water.
iv. Reptilia – Lizards, snakes, turtles, and crocodiles – dry, keratinized skin allowed reptiles to be fully terrestrial. Became the ruling terrestrial vertebrates in the Mesozoic Era (Age of Reptiles).
v. Aves – birds – first chordates adapted for flight
vi. Mammalia – mammals – 3 major groups: Monotremes (egg-laying mammals), Marsupials (pouched mammals), and Placentals (those with a placenta).