General Biology/Classification of Living Things/Eukaryotes/Animals/Phyla

Introduction to animal phyla

There currently are almost 40 recognized phyla.

Phylum — Number of Species — Common Name

Placozoa — 1
Monoblastozoa — 1
Rhombozoa — 50
Orthonectida — 50
Porifera — 9,000 — sponges (figures)
Cnidaria — 9,000 — corals (figures)
Ctenophora — 100 — comb jellies
Platyhelminthes — 20,000 — flatworms (figures)
Nemertea — 900 — ribbon worms (figures)
Rotifera — 1,800 — rotifers (figures)
Gastrotricha — 450 — gastrotrichs
Kinorhyncha — 150 — kinorhynchids
Nematoda — 12,000 — roundworms (figures)
Nematomorpha — 230 — horsehair worms
Priapula — 15
Acanthocephala — 700 — (figures)
Entoprocta — 150
Gnathostomulida — 80
Loricifera — 35
Annelida — 15,000 — segmented worms (figures)
Sipuncula — 250 — peanut worms (figures)
Echiura — 135
Pogonophora — 145 — beard worms
Vestimentifera — 8 — beard worms
Arthropoda — 957,000 — arthropods (figures)
Onychophora — 80
Tardigrada — 400 — water bears
Pentastomida — 95 — tongue worms
Mollusca — 100,000 — molluscs (figures)
Phoronida — 15
Ectoprocta — 4,500 — sessile zooids
Brachiopoda — 335 — lampshells
Echinodermata — 7000 — echinoderms (figures)
Chaetognatha — 100 — arrow worms (figures)
Hemichordata — 85 — acorn worms
Chordata — 50,000 — chordates (figures)

Phylum Porifera

The name Porifera means "pore-bearing".

This phylum is commonly called sponges. The number of species is estimated to be between 5,000 and 10,000. All are aquatic and almost all are marine.

Animals in this phyla have no true tissues, which means, for example, that they have no nervous system or sense organs. Although sponges are multicellular, they are described as being essentially at a cellular level of organization. They are sessile as adults, but have a free swimming larva.

Their bodies are porous. They are filter feeders; water flows in through many small openings (ostia), and out through fewer, large openings (osculum). They have inner and outer cell layers, and a variable middle layer. The middle layer often is gelatinous with spiny skeletal elements (called spicules) of silica or calcium carbonate, and fibres made of spongin (a form of collagen). Choanocytes are flagellated cells lining the inside of the body that generate a current, and trap and phagocytize food particles.

Their cells remain totipotent, or developmentally flexible: they can become any type of cell at any point in the sponge's development. This allows for the great regenerative power sponges have.

Sponges are an ancient group, with fossils from the early Cambrian (ca. 540 mya) and possibly from the Precambrian. Sponges often are abundant in reef ecosystems. They somehow are protected from predators (spicules? bad taste?).

Many organisms are commensals of sponges, living inside them. Some sponges harbor endosymbiotic cyanobacteria or algae (dinoflagellates, a.k.a. "zooxanthellae").

Phylum Cnidaria

Name comes from the Greek knide- meaning "nettle".

This phylum formerly known as phylum coelenterata consists of the jellyfish, hydra, sea anemones, corals, sea pens, sea wasps, sea whips and box jellyfish. There are about 9,000 species. Almost all are marine. This is another ancient group, with fossils perhaps reaching back to 700 mya.

Cnidarians exhibit radial symmetry. Their basic body plan is a sac with a gastrovascular cavity, or a central digestive system. They have one opening, which serves as both mouth and anus. The body wall has an outer ectoderm, an inner endoderm, and a variable undifferentiated middle layer called mesoglea or mesenchyme that may be jelly-like. The mesoglea is NOT considered to be true mesoderm and so the Cnidaria are described as diploblastic. Tentacles usually extend from the body wall around the mouth/anus.

There are two basic body plans: the polyp and the medusa. The polyp is sessile and attaches to substrate by the aboral end (i.e., the end away from the mouth). The medusa ("jellyfish") is a floating form, and looks like an upside-down version of the polyp. Some cnidarians only have the polyp stage, some have only the medusa stage, and others have both.

The typical life cycle of a cnidarian involves what is called "alternation of generations": an alternation between an asexual polyp stage and a sexual medusa stage.

The tentacles are armed with cnidae (or nematocysts), small intracellular "harpoons" that function in defense and prey capture. When fired, the cnidae deliver a powerful toxin that in some cases is dangerous to humans. The phylum is named after the cnidae.

Cnidarians have no head, no centralized nervous system, and no specialized organs for gas exchange, excretion, or circulation. They do have a "nerve net" and nerve rings (in jellyfish).

Many cnidarians have intracellular algae living within them in a mutualistic symbiotic relationship (Dinoflagellates = zooxanthellae). This combination is responsible for much of the primary productivity of coral reefs.

There are three main classes in the phylum

Class Hydrozoa (hydras and Portuguese man-of-war are well-known but atypical examples of this Class)
Class Scyphozoa (jellyfish)
- The medusa stage is dominant and the polyp stage often is reduced.
Class Anthozoa (sea anemones, most corals)
- No medusa (jellyfish) stage, so sexual reproduction occurs in the polyp stage in this group. The polyps also can reproduce asexually, which is how individual "corals" grow.

Phylum Platyhelminthes

Name means "flat worm"

Most members of this phylum are parasitic (flukes and tapeworms), but some are free living (e.g., planaria). There are about 20,000 species.

They are dorsoventrally compressed (i.e., "flat").

Animals in this phylum are acoelomate, triploblastic, bilaterally symmetrical, and unsegmented. Platyhelminths have a simple anterior "brain" and a simple ladder-like nervous system. Their gut has only one opening. Flatworms have NO circulatory or gas exchange systems. They do have simple excretory/osmoregulatory structures (protonephridia or "flame cells").

Platyhelminths are hermaphroditic, and the parasitic species often have complex reproductive (life) cycles.

There are four main classes of platyhelminths:

Class Turbellaria (mostly free living flatworms, e.g., planaria)
Class Monogenea (parasitic flukes)
Class Trematoda (parasitic flukes, e.g., liver fluke and the human blood fluke, Schistosoma)
Class Cestoda (tapeworms)
- Cestodes are endoparasitic in the gut of vertebrates. They do not have a mouth or digestive system.

Phylum Rotifera

The Rotifers. The name means "wheel bearing," a reference to the corona, a feeding structure (see below).

They are triploblastic, bilaterally symmetrical, and unsegmented. They are considered pseudocoelomates.

Most less than 2 mm, some as large as 2 – 3 mm.

Rotifers have a three part body: head, trunk, and foot. The head has a ciliary organ called the corona that, when beating, looks like wheels turning, hence the name of the phylum. The corona is a feeding structure that surrounds the animal's jaws. The gut is complete (i.e., mouth & anus), and regionally specialized. They have protonephridia but no specialized circulatory or gas-exchange structures.

Most live in fresh water, a very few are marine or live in damp terrestrial habitats. They typically are very abundant. There are about 2,000 species.

Parthenogenesis, where females produce more females from unfertilized but diploid eggs, is common. Males may be absent (as in bdelloid rotifers) or reduced. When males are present, sexual and asexual life cycles alternate. Males develop from unfertilized haploid eggs and are haploid. Males produce sperm by mitosis which can fertilize haploid eggs, yielding a diploid zygote that develops into a diploid female. Sexual reproduction occurs primarily when living conditions are unfavorable.

Most structures in rotifers are syncytial ("a mulitnucleate mass of protoplasm not divided into separate cells," or "a multinucleated cell") and show eutely (here, "constant or near-constant number of nuclei").

Phylum Nematoda

Name from the Greek for "thread".

This phylum consists of the round worms. There are about 12,000 named species but the true number probably is 10 - 100 times this!

These animals are triploblastic, bilaterally symmetrical, unsegmented pseudocoelomates. They are vermiform, or wormlike.

In cross-section, they are round, and covered by a layered cuticle (remember this cuticle !!). Probably due to this cuticle, juveniles in this phylum grow by molting. The gut is complete. They have a unique excretory system but they lack special circulatory or gas-exchange structures. The body has only longitudinal muscle fibers. The sexes are separate.

Nematodes can be incredibly common, widespread, and of great medical and economic importance. They are parasites of humans and our crops. They can live pretty much anywhere.

In one rotting apple, there can be up to 90,000 nematodes, and in one tablespoon of coastal mud, there can be 236 species of nematodes!

Nematodes can be free living or important parasites of our crops, or of humans and other animals. They have become very important in development studies, especially the species Caenorhabditis elegans, presumably due to its small size and constancy of cell number (eutely - 959 cells in C. elegans).

Phylum Annelida

Name means "ringed", from the Greek annulatus.

This phylum consists of earthworms, leeches, and various marine worms given many different names (e.g., sand worms, tube worms). There are about 12,000 - 15,000 species.

Animals in this phylum are triploblastic, bilaterally symmetrical, segmented coelomates. Development is typically protostomous. They have a complete circulatory system, and a well-developed nervous system. Typically, each segment has paired epidermal "bristles" (setae or chaetae).

Most are marine but they are successful occupants of almost anywhere sufficient water is available. They can be free living, parasitic, mutualistic, or commensalistic.

Major advances of this phylum include the true coelom, segmentation, both longitundinal and circular muscles, a closed circulatory system and, for most, a more advanced excretory system (metanephridia).

There are three main classes of Annelids

Class Oligochaeta (earthworms)
Class Polychaeta (marine worms, fan worms, sand worms, paddle worms)
Class Hirudinea (leeches)

Phylum Arthropoda

Name means "jointed feet".

This phylum consists of spiders, ticks, mites, insects, lobsters, crabs, and shrimp, and is the largest of all the phyla. So far, over 1 million species have been named, and it is likely that the true number out there is 10 - 100 times greater.

This phylum also includes the extinct trilobites, which were prevalent in the Paleozoic era. Because of their exoskeletons, these animals fossilized well and over 4000 species have been named.

These animals are triploblastic, bilaterally symmetrical, segmented, protostome coelomates. The coelom is generally reduced to portions of the reproductive and excretory systems. They have an open circulatory system.

The most notable advancement of this phylum is a rigid exoskeleton. It has major implications in these organisms' locomotion, flexibility, circulatory systems, gas exchange systems, and growth. It also was partially responsible for the ability of the arthropods to move on to land.

There are several major groupings of arthropods:

Major subgroups include:

The chelicerates (eurytperids, horseshoe crabs, scorpions, spiders, ticks) have clawlike feeding appendages. They lack antennae and usually have simple eyes.
The Trilobites...they get their own grouping
The uniramians (centipedes, millipedes, insects) have one pair of antennae and unbranched (uniramous) appendages.
The crustaceans (crabs, shrimp, lobsters, barnacles and many others) have two pairs of antennae and branched (biramous) appendages.

Major Classes Include

Class Arachnida (mites, scorpions, spiders, ticks)
Class Diplopoda (millipedes)
Class Chilopoda (centipedes)
Class Insecta (insects)
Class Crustacea (crabs, crayfish, lobsters, shrimp)

Phylum Mollusca

Name means "soft".

This phylum consists of snails, slugs, bivalves, chitons, squids, octopus, and many others. About 110,000 species

All molluscs have a similar body plan:

A muscular foot, usually used for movement.
A visceral mass, containing most of the internal organs.
A mantle, a fold of tissue that drapes over the visceral mass and secretes the shell, if present.
Most have a radula, or a rasping organ to scrape food.

Molluscs are bilaterally symmetrical, or secondarily asymmetrical. They are coelomates, but the coelom generally has been greatly reduced; the main body cavity is a hemocoel. Development is typically protostomous. The gut is complete with marked regional specialization. Large, complex, metanephridia (excretion).

Many molluscan life cycles include a trochophore larva. This stage also is characteristic of annelids.

There are several major classes of molluscs:

Class Polyplacophora (chitons)
Class Gastropoda (snails, limpets, conchs, slugs, nudibranchs)
Class Bivalvia (clams, mussels, scallops, oysters)
Class Cephalopoda (squids, octopus, nautilus, cuttlefish)
Class Scaphopoda (tusk shells)

Phylum Echinodermata

Name means "spiny skin"

This phylum consists of sea stars, brittle stars, sea urchins, and sea cucumbers.

Echinoderms are mostly sessile or very slow moving animals.

As adults, they are radially symmetrical, but in the larval stage, they are bilaterally symmetrical. They are considered deuterostomes.

Echinoderms are unique in that they have a water vascular system composed of a system of fluid-filled canals. These canals branch into the tube feet, which function in feeding, locomotion, and gas exchange.

There are six major classes of echinoderms:

Class Asteroidea (sea stars)
Class Ophiuroidea (brittle stars)
Class Echinoidea (sea urchins, sand dollars)
Class Crinoidea (sea lilies and sea feathers)
Class Holothuroidea (sea cucumber)

Phylum Chordata

Name means "the chordates", i.e., these animals have a notochord at some stage in their lifecycle.

This phylum consists of tunicates, lancelets, and the vertebrates.

There are four major features that characterize the phylum Chordata.

A notochord, or a longitudinal, flexible rod between the digestive tube and the nerve cord. In most vertebrates, it is replaced developmentally by the vertebral column. This is the structure for which the phylum is named.
A dorsal hollow nerve cord which develops from a plate of ectoderm that rolls into a tube located dorsal to the notochord. Other animal phyla have solid nerve cords ventrally located. A chordate nerve cord splits into the central nervous system: the brain and spinal cord.
Pharyngeal slits, which allow water that enters through the mouth to exit without continuing through the entire digestive tract. In many of the invertebrate chordates, these function as suspension feeding devices; in vertebrates, they have been modified for gas exchange, jaw support, hearing, and other functions.
A muscular, postanal tail which extends posterior to the anus. The digestive tract of most nonchordates extends the length of the body. In chordates, the tail has skeletal elements and musculature, and can provide most of the propulsion in aquatic species.

Chordates have a segmented body plan, at least in development. This segmentation evolved independently from the segmentation of annelids.

Three subphyla make up the phylum Chordata:

Subphylum Urochordata (tunicates): the adults are enclosed in a tunic made of a carbohydrate much like cellulose. They squirt water out of an excurrent siphon. Urochordates are characterized by errant (mobile and active) larvae and sessile adults. All are filter feeders. The only "chordate" characteristics retained in adult life are the pharyngeal slits. Larval urochordates look more like adult cephlochordates & adult vertebrates than adult urochordates.
Subphylum Cephalochordata: Cephalochordates are known as lancelets because of their blade-like shape; they are also known as amphioxus. They are marine animals and usually live on the bottom, but can swim.
Subphylum Vertebrata (vertebrates) ...

Formally, the phyla Urochordata and Cephalochordata are considered invertebrates.

Subphylum Vertebrata

Vertebrata refers to the presence of vertebrae and a vertebral column.

This subphylum includes most of the animals with which most people are familiar.

Vertebrates show extreme cephalization.

The notochord generally is replaced by the cranium & vertebral column in adults.

Neural Crest Cells

Later in development, these give rise to many cells of the body, including some cartilage cells, pigment cells, neurons & glial cells of the peripheral nervous systems, much of the cranium, and some of the cells of the endocrine system.

Some scientists would like to classify the neural crest as the fourth germ layer.

Neural crest cells come from the dorsal edge of the neural plate, thus ectoderm.