Link Home

Class Home Page
Monaco Educational Service © 2001



Site Information


Animal Phyla



The term zoology originates from the Greek term "zoo" meaning animal. Zoology is the study of animals. A person who studies animals is called a zoologist. With millions of species to study scientists have specialized along phylogenetic lines. that is to say they may concentrate in an area like invertebrates, fish, insects, bees, etc. (Worksheet: Fields of Study in Life Science)

Animals are multicellular, eukaryotic, heterotrophic organisms that lack cell walls. Most are capable of movement as adults. They are said to be motile. Locomotion or movement is often used in identifyiing animals. While most animals are motile, some can fool an observer into believing they are plants. Most of these live in the ocean. The sea lilies, sea fans, barnacles, and sea anemones are examples of sessile animals. As adults they have little or no movement.

The number of animal phyla differ depending on the who's classification scheme is used. In general this site employes the scheme of Margulis and Schwartz in their book... Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth, 2nd Edition. They have classified the animal Kingdom into 33 phyla.

The termsinvertebrates, animals without backbones, and the vertebrates, animals with a backbone are also used. Nearly 90% of all animals are invertebrates and most of them are insects in the phylum arthropoda.


Internet resources

Media Watch --- Outstanding. Monthly program guide to TV shows in science. Links to TV channels, Net Material, Internet Specials and Radio. ---

Virtual Dissections --- Links to 9 different animal dissections on line. --- 

Zoo Labs --- Excellent source of photos for animal labs including slides. ---


Metazoa --- Produced by Arizona University. Phyla and classes including representative photographs.

BIOSIS --- Homepage. Links to plant, animal and microbial systematics. Names and examples from phylum to orders. Search. Index to namems. Educator and student guides. ---

Introduction to Metazoa --- Produced by the University of Tennessee at Martin. Part of an exercise in the phylogeny of plants and animals. ---

History of Life --- University of California at Berkely. Table of systematics. ---

UCMP Museum of Palenothology --- Home Page. University of California at Berkely. Outstanding site for studies of geological time, evolution, and systematics (classification) ---

Vertebrates and Invertebrates ---

Invertebrates ---


Vocabulary can be a problem because the terms used are usually derived from the Greek and Latin languages. Before looking at Table 2 check learn the following prefixes and root words. They will help you to define the terms and thus have a better understanding of what the groups have in common.

Table 1: Prefixes and root words

a = without

deutero = second

meta = many

proto = before, first

schizo = splitting

zoo = animal

coelom = cavity

eu = true

para =

pseudo = false

stoma = mouth, opening

enteron = gut

bi = two

lateral = side

endo = inside

ecto = outside

meso = middle

derma = dermal, skin

di = two

por = pores

ifera = to bear

echino = spiny

arthro = jointed

poda = foot


Phylogeny refers to the evolutionary relationships among species. They are usually shown in tree diagrams. The following table shows the evolutionary relationships between the major phyla of animals. For more detailed information click on the following websites

More on phylogenetic systematics than you ever wanted to know.


*Table 2: Classification of the Metazoa (Multicellular Animals) Kingdom


The grouping of the phyla in this scheme are based on the following characteristics:


Major Phylum

Coelom formation

Body plan


Tissues and organs present

Protostomia - Schizocoelous body cavity


Spiders, crabs, scorpions, insects, shrimps


Segmented worms


Snails, slugs, bivalves, squids, octopus

Deuterostomia - Enterocoeleous cavity


Truncates, Lancelets, Vertebrates


Acorn worms


Sea stars, sea urchins, sand dollars, sea cucumbers




Acoelomata - No cavity between body wall and organism




Ribbon, proboscis worms

Pseudocoelomata - Embryonic blastocoel persists as body cavity.



Eucoelomata - Body cavity lined with mesodermal tissues




Bilateria - Body with bilateral symmetry



Radiata - Body with radial symmetry


Anemone, jellyfish, coral


Comb jellies


Eumetazoa - Tissues, organs, and digestive track


Parazoa - poorly defined tissues and no organs




Primitive metazoan

*Table adapted from Zoology, 1997. BarCharts Inc.

Print Copy of Table 1 - (Color Version) (Black and White)



Metazoa means "many celled animals". Organisms with many cells have some distinct advantages. They grow larger in size, have better mobility, maintain better internal homeostasis and are relatively independent of their environment. These advantages permit them to adapt to their environment. They are the result of a "division of labor" where cells specialize to carry out various functions. In doing so the individual cells become dependent on other cells. The down side to multicelluarity is that the failure a group of cells to carry out their function can result in the death of the organism.


Symmetry refers to balance. In nature there are three kinds of symmetry. Spherical, radial, and bilateral. The absence of symmetry is called asymmetry. Organisms in the animal kingdom with symmetry exhibit either radial or bilateral symmetry.

In radial symmetry the organism exhibits a circular arrangement with numerous planes that create identical parts. Radial symmetry is analogous to a pie in which the organism may be cut in several planes with each part (piece of pie) being identical. A organism with radial symmetry exhibits no left or right sides. It has a top (oral surface) and a bottom (aboral surface). Animals in the phyla cnidaria and echninodermata exhibit radial symmetry.

In bilateral symmetry a organism can be divided in only one plane producing two mirror image halves. (Note that we are only referring to the external surface when discussing symmetry.) This division produces a left and right side. It is found in animals with cephalization (concentration of sensory organs in the head). These animals move in the direction of their head.

Anatomical terms - the following terms are used to describe the position of structures relative to other structures or locations on the body. They often have a counter part with an opposite or converse meaning.

  • Anterior - front, in front of (head)
  • Posterior - after, behind, following, in the rear
  • Ventral - toward the bottom, toward the belly
  • Dorsal - near upper surface, toward the back
  • Inferior - below, under
  • Superior - above, over
  • Distal - away from, farther from the origin
  • Proximal - nearer, closer to the origin
  • Lateral - toward the side, away for the
  • Medial - toward the midline, middle, away from the side
  • Caudal - toward the back, toward the tail
  • Crainal, Rostral - toward the front


All methods of classification have a strong basis in morphological (structure) similarities. However, the presence of wings does not mean two organisms are closely related (i.e. insects and birds). The study of an organism's embryology (development of embryos) provides taxonomists with a power tool in constructing relationships. The development of all animals follows a similar pattern beginning with a fertilized egg cell known as a zygote. As each group of animals evolved it added more complexity to its development while repeating the history of its ancestors. This tendency was studied by Ernst Haeckel in 1866 referred to as "ontogeny recapitulates phylogeny".

Zygote ---> 2 cells ---> 4 cells ---> 8 cells ---> 16 cells ---> Morula (solid ball of cells) ---> Blastula [BLAST-choo-la] w/Blastrocoel chamber ---> Gastrula w/Blastopore and Archenteron (embryonic gut)


Germ Layers

During the gastrula stage three germ layers develop. All other tissues come from these layers. They are the endoderm (inside skin), mesoderm (middle skin), and ectoderm (outside skin).

  • Ectoderm - outside skin tissues
  • Endoderm - inside tissues lining the gut
  • Mesoderm - muscle tissue, internal linings, blood vessels, blood, skeleton

Animals that produce all three layers are known as triploblastic. Some animals only produce two layers, endoderm, and ectoderm. These are said to be diploblastic.

The following table shows the development processes characteristic of protostomes and deuterostomes.

Table 3: Characteristics of a protostome vs. a deuterostome.




Cleavage pattern



Developmental fate of cells



Coelom formation



Blastopore fate



Cleavage patterns

  • In spiral cleavage new cells develop at the junctures between previous cells. This occurs in protostomes.
  • In radial cleavage new cells are placed beside or on top of previous cells. This occurs in dueterostomes.

Developmental fate of cells

  • Determinate The fate of the cells is determined early on during development. If these cells are separated early on they will not develop into an entire organism. This occurs in protostomes.
  • Indeterminate The fate of the cells is determined relatively late. If the cells are separated early they can be developed into a new organism. This produces "twinning" (genetically identical individuals) in animals. This occurs in dueterostomes.

Coelom formation refers to the development of a body cavity lined with mesodermal tissues.

  • Schizocoely - The coelom develops from a split in the mesoderm. This occurs in protostomes
  • Enterocoely - the coelom develops from the archenteron (embryonic gut). This occurs in dueterostomes.

Blastropore fate - The blastropore is the opening into the archenteron (embryonic gut) that developed in during gastrulation (formation of the gastrula). Some animals have an incomplete digestive track (one opening) but others have a complete digestive track (two openings). In animals with a complete digestive tract the blastopore may become a mouth or an anus depending on the genetic nature of the organism.

  • Mouth - The blastropore becomes an anus and a second opening called the mouth forms in the embryo. This is typical of protostomes.
  • Anus - The blastopore becomes a mouth and a second opening called an anus forms in the embryo. This is typical of deuterostomes.

Body plans

  • Acoelomates [ay-SEE-luh-MATE] - Means without a coelom. There is no cavity between the body wall and organs.
  • Pseudocoelomates - Means false coelom. The blastocoel remains and forms a body cavity.
  • Coelomates - Have a true coelom [SEE-lum] which is a body cavity lined with mesodermal tissues.

    Enrichment: What is a lophophorste? Give examples.

Larva Development

  • Indirect development means the organism has an intermediate form or larva that does not look like the adult.
  • Direct development has no larval stage. The young look like the adult when they are hatched such as silverfish and grasshoppers.

Aquatic free swimming invertebrates have one of three types of larva which are useful in identifying evolutionary relationships among the phyla. The plannula is free swimming, ciliated, with two body layers. They settle to the bottom and develop into a polyp colony. Trochophores are free swimming and bilaterally symmetrical. Mollusca and annelida phyla produce trochophores.Bipinnaria (dipleurula) larva are free swimming with a distinctive developmental pattern. The echinoderm phyla produce them.


*List of 33 Animal Phyla according to Margulis and Schwartz

1. Placozoa

10. Rotifera

19. Brachiopoda

28. Arthropoda

2. Porifera

11. Kinorhyncha

20. Mollusca

29. Pogonophora

3. Cnidaria

12. Loeicifera

21. Priapulida

30. Echinodermata

4. Ctenopora

13. Acanthocephala

22. Sipuncula

31. Chaetognatha

5. Mesozoa

14. Entoprocta

23. Echiura

32. Hemichordata

6. Platyhelminthes

15. Nematoda

24. Annelida

33. Chordata

7. Nemertina

16. Nematomorpha

25. Tardigrada

8. Gnathostomulida

17. Ectoprocta

26. Pentastoma

9. Gastrotricha

18. Phoronida

27. Onychophora

*Major Animal Phyla

Primitive vs. Advanced Animals

Most Primitive ---------------------------------------------------------------------------> Most Advanced

Body Plan


Sac plan.

Incomplete digestive system (one opening)


Complete digestive system (two openings)

Tube-within-a-tube with specialization of parts

Schizocoely and Enterocoely


Asymmetrical (None)



Bilateral with cephalization

Germ layers


2 (diploblastic)

3 (triploblastic)

3 (triploblastic)

Level of organization




Organ systems





True coelom





Segmentation with specialization of parts






Protostomes: Annelida, Arthropoda, Mollusca

Deuterostomes: Echinodermata, Hemicordate, Chordata,

All Rights Reserved © 2001

Top of Page