Laboratory 1


Parazoa, Radiata, Bilateria (Phyla of Unknown Relationship)


Porifera, Placozoa, Cnidaria, Ctenophora, Mesozoa, Platyhelminthes, Gastrotricha, Gnathostomulida, Micrognathozoa, Rotifera, Ciliophora, Chaetognatha


Follow the links to learn more about the highlighted animal groups and to see illustrations and photographs. Complete the exercise at the end of the lab, and submit by e-mail or in writing.


This lab focuses on the more primitive invertebrate phyla including the Parazoan Porifera (sponges) with an aggregate body plan, the Radiata phyla Placozoa, Cnidaria and Ctenophora with radially symmetrical blind-sac body plans, and the Bilateria phyla of unknown relationship Mesozoa, Platyhelminthes (Flatworms), Gnathostomulida, Gastrotricha, Rotifera, Micrognathozoa, Ciliophora, and Chaetognatha.


The Parazoa


  1. Porifera – The Porifera are the sponges. There are approximately 5,000 species of sponges that are found mostly in marine environments. About 100 species of sponges occur in freshwater. Sponges are sessile (most species do not move). Sponges are normally found attached to rocks, shells, submerged timbers, or coral or other hard substrates.


     Tropical tube and encrusting sponges, Saba.        Barrel sponge with a Coney (fish) in it, Saba.


            Pictures courtesy of BIODIDAC


Classification: Sponges are divided into three classes: Hexactinellida (glass sponges) Calcarea (calcareous sponges), and the Demospongia (the largest group of sponges). (See more pictures)


Morphology: Sponges range between 1 cm and 2 m in size. They consist of a lose aggregation of cell types that form two layers of cells separated by a gelatinous protein matrix (the mesohyl).  The surface of sponges is perforated by many small openings through which water is drawn by flagellar action. The water exits the sponge through a larger opening known as the osculum. Water flows into an interior chamber (the atrium or spongocoel) the surface of which is lined with choanocytes (collar cells) that trap and filter food from the water. Another motile cell type, amoebacytes, are involved in digestion and distribution of nutrients among cells.   Sponges may be irregular in shape, columnar, branching, or vase shaped.


Fossil record: Late Precambrian (650-543 my bp [million years before present])


            Read more about Porifera on the UCMP website.

            Porifera in the Tree of Life website

            Porifera on the Paleos website.

            Porifera on the Animal Diversity Web.

            More images of Porifera on BIODIDAC.


The Radiata


2. Placozoa – The Placozoa are an enigmatic small group of animals that are sometimes placed in the Parazoa along with the Porifera.  These animals were discovered in an aquarium, and have never been observed in there natural habitat. Only two species have been described.


Drawing of Placozoa


Pictures courtesy of BIODIDAC


Classification: With only two described species the Placozoa has not been divided into subtaxa. (See drawing)


Morphology: The Placozoa are small (3mm or less in diameter). They have two cell layers that are both ciliated. No other distinguishing morphological features have been reported.


            Fossil record: No fossils have been found.


             Read more about the Placozoa on UCMP website.

            Placozoa in the Tree of Life website.

            Images of Placozoa on BIODIDAC.

            Placozoa on Angelfire.


3. Cnidaria – The Cnidaria are comprised by the anemones, corals, jellyfish, hydra, and box jellies. The Cnidaria are largely marine with about 9,000 species, but only about 100 freshwater species of hydroids.


Scyphozoa- Aurelia- Medusa of the                           Anthozoa- Telia anemone

common jelly fish, Aurelia.                                                     Hudson's Bay near Churchill  



 Anthozoa- Chancanab Reef, 45 ft.                     Cubozoa- Diagram showing the major

 Cozumel Mexico                                                     features of the adult cubozoan.



Hydrozoa- Obelia- Life cycle of Obelia (Hydrozoa),           Scyphozoa- Aurelia- Diagram of the showing the planula, colonial polyps

and the free swimming medusa stage.                                 and the free swimming medua stage.


Pictures courtesy of BIODIDAC


Classification: The Cnidaria are divided into four classes: the Anthozoa (anemones, true corals, and sea pens), the Cubozoa (the box jellies), the Hydrozoa (hydroids, fire corals, and other medusoid forms), and the Scyphozoa (the true jellyfish). 


Morphology: The Cnidaria usually have two tissue layers, a gastrovascular cavity, and a single body opening that serves as mouth and anus.  Cnidaria can be solitary or colonial and take on a polypoid and medusoid form, showing an alternation of asexual (polypoid) and sexual (medusoid) forms in some groups. The Cnidaria have nematocysts which are organelles specialized for prey capture. They contain a coiled tubular thread with bards that may contain poisons. The Cnidaria are largely “sit and wait” predators that capture prey with their nematocysts. 


            Fossil Record: late Precambrian (550 my bp).


            Read more about the Cnidaria on UCMP website.

            Cnidaria on the Tree of Life Website.

            Cnidaria on the Paleos website.

            Cnidaria on the Animal Diversity Web.

            Images of Cnidaria on BIODIDAC.

            Cnidaria on Angelfire.

            Cnidaria on Marine Invertebrates in the Plankton web site


4. Ctenophora – The Ctenophora are the comb jellies. They are entirely marine in distribution and consist of only about 100 species.


General body plan of Ctenophora

Pictures courtesy of BIODIDAC


Classification: The Ctenophora is divided into three classes or subclasses: the Tentaculata, the Typhocoela, the Cycloela, and the Nuda.


Morphology: The Ctenophora have two tissue layers, a mouth and two annual pores. They capture prey using sticky cells called collobasts.  The eight rows of cilia on the body surface beat rhythmically to transport food to the mouth and to propel the animal through the water.


            Fossil Record: Cambrian (545-525 my bp)


            Read more about the Ctenophora on UCMP website.

            Ctenophora on the Tree of Life Website.

            Ctenophora on the Paleos website.

            Images of Ctenophora on BIODIDAC.

            Ctenophora on Marine Invertebrates in the Plankton web site

More on Ctenophora


The Bilateria – Phyla of Unknown Relationship


A tentative phylogeny based on 18s Ribosomal rRNA


5. Mesozoa – Are small worm-like animals that live as parasites of other marine invertebrates.


General body plan of Mesozoa

Pictures courtesy of BIODIDAC


Classification: The Mesozoa are divided into two Classes: the Rhombozoa and the Orthonectida.


Morphology: Mesozoans are worm-like parasites of marine invertebrates that consist of only 20-30 cells in two cell layers. Locomotion is achieved by the action of cilia.


            Fossil Record: Unknown from fossils


            Mesozoa on the Tree of Life Website.

            Images of Mesozoa on BIODIDAC.

            Mesozoa on Biology Cabinet web site


6. Platyhelminthes – The Platyhelminthes are the flatworms. This group includes the Cestoda (tapeworms), the Monogenea (flukes), and the Trematoda (flukes), which are parasitic, and the free-living Turbellaria. The tapeworms and flukes are the causes of many human diseases. Recent suggestions are that the Platyhelminthes is actually polyphyletic. There are about 12,200 species of flatworms described worldwide.






Pictures courtesy of BIODIDAC


Classification: The Platyhelminthes is divided into four Classes: the Turbellaria, the Trematoda, Monogenea, and the Cestoda.


Morphology: The Platyhelminthes are bilaterally symmetrical and have no other body cavity than the gut. Smaller species have no anus.


            Fossil Record: Unknown


            Read more about the Platyhelminthes on UCMP website.

            Platyhelminthes on the Tree of Life Website.

            Platyhelminthes on the Animal Diversity Web.

            Images of Platyhelminthes on BIODIDAC.

            Platyhelminthes on the Glasgow University Museum website.

            Images of Platyhelminthes on BioImages.


7. Gnathostomulida – The Gnathostomulida are tiny marine worms also know as “jaw worms.” They occur to a depth of several hundred meters and interstitially between sand grains. They were first described in 1956. Approximately 80 species have been described worldwide.

General body plan of Gnathostomulida

Pictures courtesy of BIODIDAC


Classification: The Gnathostomulida is divided into two Classes: the Filospermoidea and the Bursovaginoidea.


Morphology: Gnathostomulida have a digestive system that is a blind sac with no anus. Most species are hermaphroditic. They have one cilia per cell.


            Fossil Record: Unknown from fossils


            Gnathostomulida on the Tree of Life Website.

            Gnathostomulida on the Extinction and Biodiversity website.

            More Gnathostomulida.

            Images of Gnathostomulida on BIODIDAC.


8. Gastrotricha – The Gastrotricha are small (less than 3mm) free-living aquatic worms that are most common in or on the surface of sediments where they can be among the most abundant animals. There are approximately 690 species of Gastrotricha worldwide.


General body plan of a Gastrotricha

Pictures courtesy of BIODIDAC


Classification: The Gastrotricha is divided into two orders: the Macrodasyida and the Chaetonotida.


Morphology: Small predaceous worms. Bodies flattened ventrally and the posterior is sometimes forked. Cilia are restricted to the ventral surface. Gastrotricha are hermaphroditic.


            Fossil Record: Unknown from fossils


            Gastrotricha on the Tree of Life Website.

            Images of Gastrotricha on BIODIDAC.

            Gastrotricha on the Glasgow University Museum website.

            Gastroticha World Portal

            Gastrotricha on BioImages


9. Rotifera – Small, microscopic organisms found in freshwater, moist soil, and in thin films of water on mosses, lichens, mushrooms, and on aquatic insect larvae. There are about 1852 species of Rotifera worldwide. Also called “wheel animacules.”


Asplanchna sp. Whole mount                                          

Transparent cuticle permits viewing

internal organs.                                        Live rotifer with ciliary ring on head.


Pictures courtesy of BIODIDAC


Classification: The Rotifera is divided into three classes: the Monogonota, the Bdelloidea, and the Seisonidea. More recently, the Acanthocephala (the spiny-headed worms) have also been considered part of the Roifera.


Morphology: Rotifers have body cavities partially lined by mesoderm. They have a complete digestive track including a moth an anus. They have a horseshoe shaped crown of cilia on the head, which draws water into the mouth. Rotifers then filter this water to remove food particles. The Rotifer foot has a cement gland, which allows to animals to attach to substrates.


            Fossil Record: Oldest fossil from the Eocene (55-33 my bp)


            Read more about the Rotifera on UCMP website.

            Rotifera on the Tree of Life Website.

            Images of Rotifera on BIODIDAC.

            Rotifera on Angelfire.

            Rotifera on the Glasgow University Museum website.


10. Micrognathozoa – A single species discovered from a cold spring in Antartica that cannot be placed in any other phylum. Appears to have rod-like structures in the jaws similar to Rotifers and Gnathostomulida.


Classification: Only a single species described (Limnognathia maerski). Potentially allied with the Rotifers and Gnathostomulida.


Morphology: A small worm-like animal with cilia that serve sensory purposes and others that are used in transport of food particles toward the mouth, and for locomotion. The most distinctive structure are the jaws.


Fossil Record: Unknown from fossils


More on Micrognathozoa

Micrognathozoa on the Tree of Life web site


11. Ciliophora – Consists of a single species discovered in 1995 attached to the lip of a lobster. This organism has a complicated life cycle with both sessile and free living stages, and reproduces both sexually and asexually.


Classification: Only a single species described (Symbion pandora).


Morphology: The sessile stage is only 350 mm long. S. pandora is attached by an adhesive disk to the lip of the Norway lobster. It feeds with a mouth surrounded by a ring of cilia. Excretion occurs via an anus that is located near the mouth.


Fossil record: Unknown from fossils.


More on the oddities of Ciliophora

Ciliophora on BIODIDAC

Ciliophora on the Tree of Life web site


12. Chaetognatha – The Chaetognatha are the arrow worms. They can be very abundant in the plankton. They are predators of other planktonic forms including larval crustaceans. They are transparent and shaped like an arrow.


Head with grasping spines                                                                

of Chaetognatha                                                  Sagitta sp. (Chaetognatha)


Pictures courtesy of BIODIDAC

Classification: The Chaetognatha is divided into two Classes: the Phragmophora and the Apragmophora.


Morphology: Chaetognatha are transparent and no more than a few millimeters in length. They have grasping spines on the head for prey capture. Chaetognatha have a mouth and an anus, and are hermaphroditic.


            Fossil Record: Precambrian (550 my bp)


Read more on Chaetognatha

            Chaetognatha on the Tree of Life Website.

            Images of Chaetognatha on BIODIDAC.

            Chaetogonatha on Invertebrate in the Plankton web site


Lab 1 Exercise


Biologists use dichotomous keys to identify animals and plants. Dichotomous keys use couplets of questions to separate groups of organisms by important traits or characters. For example a dichotomous key to separate and identify a thumb tack, a paper clip, and a rubber band might be like this.


1a. Metallic in appearance                                                              go to 2

1b. Non-metallic in appearance                                                       must be a rubber band


2a. Has a sharp point                                                                      must be a thumb tack

2b. Has no sharp points, consist of

rounded metal wire                                                                   must be a paper clip


If we needed to separate more objects (animal groups) we would need to use more couplets of questions.



Task #1 - Try to construct a dichotomous key for as many of the 12 Phyla discussed in this lab. You will need to consult the websites linked to in this lab web page, and possibly others. Try using Google or another search engine to find additional information. Anytime the material you find says “this trait is unique to this phylum” you can use that trait to separate that phylum from others.  Build asset of couplet questions that


Task #2 – Search for other links on the web that provide useful information on the 12 Phyla discussed in this lab. Try to find 3 more links and submit those links to be added to the web page.


Task #3 - Try to find any information on the number of described species and the fossil record for any of the Phyla discussed in this lab for which that information was not provided. Do you find any mention of extinct taxa (classes or orders) from the 12 Phyla discussed in this lab?