Animal Phyla

Under NAQT rules, unless the question states otherwise, both Latin names (Mollusca) or Anglicized names (molluscs) are acceptable for a given taxon. Phyla of plants, algae, fungi, and sometimes bacteria are often referred to as “divisions.” Estimates of phylal diversity vary. Because many invertebrates are inconspicuous, all estimates are probably low. Unless stated otherwise, numbers represent an estimate of the number of species that have been named.

Porifera

(5,000 species): The sponges are all water-dwellers (98% marine, 2% freshwater), and are sometimes classified separately from other animals because of their asymmetric bodies and lack of distinct tissues. They are sessile (immobile) except in early dispersing stages, and collect food particles via the sweeping motions of flagellated cells called choanocytes (koh-AN-oh-“sites”).

Cnidaria

(10,000 species): Also called Coelenterata (see-LEN-tur-AH-tuh), these develop from a diploblastic (two-layered) embryo, and have two separate tissue layers and radial body symmetry. Many have two life stages, the mobile, usually bell-like medusa and the sessile polyp. All have nematocysts, or stinging cells, for capturing prey, and some can inflict painful stings on swimmers. Examples include the hydras, sea anemones, corals, jellyfishes, and Portuguese man-o-war (which is actually an aggregation of colonial []).

Platyhelminthes

(15,000 species): The flatworms are the most primitive phylum to develop from a triploblastic (three-layered) embryo. They have bilateral body symmetry, and are acoelomate (lacking a true body cavity), so that the space between the digestive tract and the body wall is filled with tissue. As the name implies, they are generally flat-bodied. They have a true head and brain, but the digestive system has only one opening, which functions as both mouth and anus. Most are hermaphroditic. This phylum includes parasites such as the tapeworms and flukes, as well as free-living (i.e., non-parasitic) organisms such as the planarians.

Nematoda

(15,000 species): The roundworms are unsegmented worms that live in a variety of habitats. They are pseudocoelomate; the three tissue layers are concentric, but the body cavity is not lined with tissue derived from the mesoderm (middle embryonic layer). They include both free-living and parasitic species; human parasites include hookworms and the causative agents of elephantiasis, trichinosis, and river blindness. Soil ones may be crop pests, while others are beneficial predators on other plant pests. The species Caenorhabdis elegans is a common subject in genetics and developmental-biology labs.

Annelida

(11,500 species): These are segmented worms and represent the first lineage of truly eucoelomate animals, meaning their body cavities are lined with tissue derived from the embryonic mesoderm. These classes include the marine Polychaeta, as well as the mostly terrestrial Oligochaeta (including the earthworms, Lumbricus) and the mostly aquatic Hirudinea, or leeches. Characteristics of these include nephridia (kidney-like structures), blood vessels, and, in some classes, hermaphroditism.

Arthropoda

(over 800,000 species described; estimates of actual diversity vary but go as high as 9 million species): The most diverse and successful animal phylum on earth (incorporating about 75% of all described animal species), these are characterized by jointed legs and a chitinous exoskeleton. Like annelids, they are segmented, but unlike annelids, their segments are usually fused into larger body parts with specialized functions (such as the head, thorax, and abdomen of an insect). They are often divided into four subphyla: Uniramia (insects, centipedes, millipedes); Chelicerata (arachnids, sea spiders, horseshoe crabs); Crustacea (shrimps, lobsters, crabs, crayfish, barnacles, pillbugs), and Trilobitomorpha (the trilobites, now extinct).

Mollusca

(50,000 species): These are second in diversity only to the arthropods. Body plans within this phylum are diverse, but general characteristics include a soft body covered by a thin mantle, with a muscular foot and an internal visceral mass. There are two fluid-filled body cavities derived from mesodermal tissue: a small coelom and a large hemocoel that functions as an open circulatory system. Many have a shell composed of calcium carbonate and proteins, secreted by the mantle. Familiar groups within this phylum include the classes Gastropoda (slugs, snails), Bivalvia (clams, oysters, scallops), and Cephalopoda (nautilus, squids, octopi).

Echinodermata

(6,500 species): Characteristics of this phylum include an endoskeleton composed of many ossicles of calcium and magnesium carbonate, a water vascular system, a ring canal around the esophagus, and locomotion by tube feet connected to the water vascular system. Unique to these is the five-fold radial symmetry obvious in sea stars (starfish), sea urchins, and sea lilies. Others, like sea cucumbers, have varying degrees of bilateral symmetry. In their body plan, a true head is absent; the anatomical terms oral (mouth-bearing) and aboral (away from the mouth) are used to describe orientation of the body surfaces. Feeding adaptations include particle feeding through the water vascular system, everting the stomach to engulf prey (sea stars), and a scraping device called Aristotle’s lantern (sea urchins).

Chordata

(44,000 species): The phylum that contains humans, this is divided into three subphyla: Urochordata, the sea squirts; Cephalochordata, the lancelets, and Vertebrata, the true vertebrates, which is the most diverse subphylum. Defining traits include pharyngeal gill slits, a notochord, a post-anal tail, and a dorsal hollow nerve cord. In vertebrates, some of these structures are found only in embryonic stages. The lancelet Amphioxus (Branchiostoma) is often used as a demonstration organism in biology labs.