UNM Biology Undergraduate Labs

Simple Animals

Useful Reading

Campbell, Biology 6th Ed - Chapters 32 & 33, pgs 633-655, 661-662

Campbell, Biology 7th Ed - Chapter 32 & 33, pgs 626-650, 655-626


Symmetry – general, structured body plan. See radial and bilateral symmetry below.

Coelom – fluid-filled body cavity surrounded by mesoderm-derived tissue.

Diploblastic – having only two layers of cells during development: the endoderm and ectoderm.

Triploblastic – having three layers of cells during development: the endoderm, ectoderm, and mesoderm.

Heterotroph – an organism which gets its energy by ingesting food.

Hermaphrodite – an individual which can produce both eggs and sperm.


In Kingdom Animalia, all taxa are eukaryotic and lack cell walls, multicellular, and generally heterotrophic. Evolutionary relationships among animals have been examined using fossil evidence, anatomy, development, and genetics. You should familiarize yourself with the major anatomical and developmental differences that define animal phyla. In this section, we will look at the simplest and earliest animals – those which have the simplest body structure and tissue organization.

The traditional phylogeny is shown below. It is based on the following anatomical / developmental traits:

The grades of animal evolution are as follows (italics – taxa described here):

1.  No true tissues (only non-specialized cells) :  Parazoa


     Porifera (sponges)


2.  True tissues (functionally-specialized cells) :  Eumetazoa


     A.  Radial symmetry, diploblastic (2 germ layers during development) :  Radiata


              Cnidaria (jellyfish, coral, anemone)

              Ctenophora (comb jellies)


     B.  Bilateral symmetry, triploblastic (3 germ layers) :  Bilateria


1)     No body cavities or blood vascular system :  Acoelomates


Platyhelminthes (flatworms)


2)     Body cavities and blood vascular system


a.  Body cavity not enclosed in mesoderm : Pseudocoelomates


     Nematoda (roundworms)

     Rotifera (rotifers)


b.  Body cavity enclosed in mesoderm : Coelomates


     Protostomes and Deuterostomes (see associated lab summaries)

This phylogeny of all animals is based on morphological differences and similarities.


Recent phylogenetic analyses based on genetic data have revealed slightly different phylogenetic relationships:

If you compare this phylogeny to the morphological phylogeny, you can see that certain relationships are similar. Parazoans are still the basal animals. In addition, the Radiata are still a monophyletic group, and the Bilateria are still a monophyletic group. However, the relationships among the acoelomates, pseudocoelomates, and true coelomates are different.

While the true phylogenetic relationships among some phyla of animals are still contested, you should remember how these groups differ in major morphology and physiology. 

Subkingdom Parazoa

Parazoa is considered the most ancestral animalian group. Parazoans have a simple anatomy and lack true tissues; that is, their cells are relatively unspecialized. They have no nerves, muscles, organs, or organ systems.

Phylum Porifera (sponges)

Sponges belong to the subkingdom Parazoa. They are sessile and aquatic. They feed by drawing water into their body cavity (spongocoel) through pores and pushing it out through the osculum. They filter-feed from this water by trapping food particles with special cells called choanocytes, or collar cells. All energy and oxygen are obtained through simple diffusion across each cell membrane (i.e. no special circulatory or respiratory system). Diffusion is also used to rid cells of waste. Small spicules, made of calcium carbonate, project out of the body surface, giving support and protection to the sponge. Sponges are hermaphrodites.



Subkingdom Eumetazoa

All animal phyla except Porifera belong to Subkingdom Eumetazoa. Eumetazoa have at least two layers of true tissues (ectoderm and endoderm); that is, they have cells which group into specialized functional groups.


- Radiata

Radiata refers to a monophyletic group of eumetazoan animals which have radial symmetry. This means they have a top and bottom, and an oral (mouth) and aboral side, but no front/back, right/left. Radiata are also diploblastic: they have two cell layers (germ layers), ectoderm and endoderm. Radiata contains two Phyla: Cnidaria and Ctenophora.

Radial Symmetry

Phylum Cnidaria (jellyfish, corals, anemones)

Cnidarians (the “c” is silent) are aquatic, radially symmetric, and have two tissue layers (diploblastic). They have no coelom, but they have a gastrovascular cavity (digestive sac) with only one opening. They have no complex organs. There are two body plan variations: a floating medusa (e.g. jellyfish), and a sessile polyp (e.g. Hydra). They feed by capturing prey using the tentacles surrounding their mouths. These tentacles have specialized cells (cnidocytes) containing stinging nematocysts. Prey are pulled into the gastrovascular cavity where they are digested. Cells obtain nutrients and get rid of wastes though simple diffusion. Their nervous system is a diffuse nerve net. They have muscle-like fibers in their two cell layers, but no true muscles. They can locomote with their tentacles. They maintain their shape using a hydrostatic skeleton.

Diagram of polyp and medusa forms of cnidarians.


Hydra (left) and a jellyfish (right)


Diagram and picture of cnidocytes and nematocysts




Bilateria refers to a monophyletic group of eumetazoan animals which first evolved bilateral symmetry. This means they have dorsal/ventral sides and anterior/posterior sides. Animals in Bilateria have various degrees of cephalization – concentration of sensory tissues at the anterior end.

Bilateral Symmetry

Bilateria are also triploblastic, rather than diploblastic. During development, they make three germ layers: the endoderm, ectoderm, and the mesoderm (between ecto- and endoderm). Phyla within the Bilateria clade are classified according to the presence and development of a coelom (body cavity):


Acoelomates :

Solid body, no cavity between gut (endoderm) and outer body. Lack a blood vascular system. 


Pseudocoelomates :

Body cavity is not completely lined with mesoderm.


Coelomates / Eucoelomates :

Body cavity is completely lined with mesoderm-derived tissue; it is a true coelom.


Phylum Platyhelminthes (flatworms)


Flatworms are acoelomates. They are mostly aquatic or parasitic. They have a branched gastrovascular cavity with only one opening (pharynx) into which they ingest and digest food. Nutrients reach cells through diffusion. Wastes diffuse from cells into the surrounding water. They have true muscles and are supported by water. Free-living flatworms (e.g. Planaria) are mobile, using cilia on their ventral surface to glide over surfaces. They generally have heads with a small “brain” (cephalization) and two ventral nerve cords. Some have eyespots on their heads. Platyhelminthes includes hermaphroditic and non-hermaphroditic taxa. Similarly, some reproduce only sexually, with others also reproduce asexually.


Classes of Platyhelminthes:

·        Class Turbellaria : free-living, e.g. Planaria

Planaria: note the branched digestive cavity

Cross-section of Planaria showing three tissue layers: epidermis (ectoderm), muscle (mesoderm), and lining of digestive tract (endoderm).

·        Classes Monogenea and Trematoda : parasitic flukes

Schistosoma mansoni life cycle and adult fluke (Trematoda)

S. mansoni miracidia and cercaria

     ·        Class Cestoda : parasitic tapeworms


whole tapeworm (left) and scolex (head; right)

Phylum Nematoda (roundworms)

Roundworms are pseudocoelomates. They are free-living or parasitic and have a complete digestive tract with two openings (mouth and anus). Nutrients are transported to non-digestive cells in the fluid of the pseudocoelom. The excretory system consists of two lateral lines which transport wastes from cells to outside of the body through excretory pores. They have longitudinal muscles which they use to move, and are supported by a hydrostatic skeleton. Their nerve cells are concentrated into nerve cords. Their body wall has a cuticle on the outside over the epidermis. Roundworms are non-hermaphroditic and reproduce sexually.



Dissection of Ascaris female


Cross-section of Ascaris female

Review Questions


-Compare these four phyla with respect to digestive modes, cell nutrient acquisition, and cell waste elimination.

-Which of the four Phyla above have tissues derived from three different germ layers?

-What gives sponges the hardness that you notice when you purchase a natural sponge at the store?

-Does a jellyfish have a brain?

-What traits make biologists think that sponges are the most ancestral of all living (extant) animals?

-Which of these phyla has neither bilateral of radial symmetry?

-What makes a roundworm’s body cavity a pseudocoelom rather than a true coelom?

-According to the old morphological phylogeny, which evolved first: acoelomates or pseodocoelomates? Does the new molecular phylogeny confirm the same order of evolution?

-Compare the locomotion patterns (i.e. directionality) of mobile radial and bilateral symmetrical animals.

-Why is concentration of nervous tissue (cephalization) associated with bilateral symmetry?