Lab Topic 19:
Animal Phylogeny: Investigating the Evolution of Body Plan

Cnidaria

Platyhelminthes

Nematoda

 

 

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Phylum Porifera 

Phylum Synopsis: Commonly known as sponges, the Latin name of the phylum derives from the numerous pores found on the body surface. The 9,000 or so species are all aquatic with most representatives living  in salt water, although about 100 species live in fresh water habitats.. These simple multicelluar animals are an offshoot of the main evolutionary paths seen in the animal kingdom and are placed in the branch Parazoa because they are composed of a more or less loose assemblage of cells rather than  the specialized tissues found in other animals.This classification is suported by modern molecular evidence. Sponges are placed into three taxonomic classes based on the chemical composition of their skeletons. Those with skeletons made of calcium carbonate are in the class Calcarea. Those with skeltons composed of silicon dioxide are in the class Hexactinellida. Those with skeletons made from protein fibers are in the Class Demospongiae. Sponges offer an interesting test of your concept of what is an animal.

The general body plan consists of two cell layers surrounding a spongocoel. The body wall is approximately two cell layers thick with a gel like substance called the meohyl in-between. The body wall is perforated by many pores and channels through which water enters the animal, passing into the spongocoel, and exiting it through a large opening, the osculum. There are no organs or appendages and digestion is intracellular ( within cells). Unique collar cells, called choanocytes, are found only in these animals. Choanocytes line internal chambers,  the spongocoel, and the beating of their flagella draws water into the animal and expels it. Small food particles suspended in the water stream are captured by the action of the collars on the choanocytes. The particles are ingested by phagocytosis and may be passed to other cells by exocytosis. Digestion is intracellular. The cells of the body are supported by secreted skeletal elements called spicules. Body shapes are varied but can be described as asymmetrical and  vase-shaped often with branching to form colonies in those species that stand alone, or encrusting in those that grow in layers on the surfaces of rocks, dead coral, or other suitable substrates. Sponges can reproduce asexually when they are fragment by water disturbances or by predator actions and pieces of the sponge grow into new individuals. They can also reproduce sexually. Most are hermaphrodites and a single individual  releases eggs and sperm. A ciliated larval stage develops from the zygote and is free swimming, allowing dispersal of the species from one location to another.
 

Body form
 

Photo taken of a whole mount  of Leucosolenia sp.  Stained to improve contrast. Size less than 1" Note basal holdfast area which attaches the sponge to  substrates.  How would describe the ssymmetry of this sponge?

Close up photo showing the osculum of a Leucosolenia. Stained to improve contrast. What is the name of the opening at the tip of the sponge? What is its function? What type of body plan does this particular sponge have?  a. asconoid b. syconoid c. leuconoid

Skeletal System
At increasing magnifications you can see the skeletal structure composed of spicules. Triaxon spicules are shown on the left. Look closely at the lower magnifications and you can see the outline of intermeshed spicules in the body wall with cells growing around the outside. These spicules are made of calcium carbonate and are secreted by special cells called amoebocytes that migrate in a gelatinous layer, called the mesohyl, between the two cell layers that make up the body wall. The cells covering the outer surface of a sponge is called a pinacoderm.

Other species of sponges have skeletons made from  a protein called spongin shown in photos below. Cells of the body  grow around these fibers and are thus supported by the protein matrix. When sponges are commercially harvested, they are dried and washed to remove the cells and the protein meshwork is sold as a natural sponge for bathing or washing cars.

Whole sponge

Close-up of sponge surface showing spongin and canals

HI magnification view of spongin fibers

  Some deep ocean species, called glass sponges, have elaborate skeletons made from silica that support the cells of the body. It boggles the mind to imagine the biochemical reactions that are required to concentrate silica from sea water and to secrete it as fibers to form the elaborate structures seen below.

Glass sponge skeleton

Close-up of glass sponge silicon dioxide spicules

 

Cellular organization
The photo below is a cross section of a sponge in the genus Scypha. Its body wall is folded back and forth on itself. A thin slice was cut and mounted on a slide. It was stained to improve viewing and then the photo was taken. By looking at it, you can see how the body wall consists of only a few cells. The resolution is not good enough to be able to see the unique collar cells (choanocytes). The illustration on the right is an artist's interpretation of a similar slide.

Section through body wall to show cellular arrangements.  About how many cells thick is the body wall?  In the diagram can you see porocytes? What is their function?  Where are the coanocytes?  Using the illustration above, describe how water passes through a sponge and how food particles are removed.  What do sponges 'eat?"

A choanocyte What is the function of this type of cell in sponge?  What other animals have choanocytes?

Test your knowledge:

Below are three photos of a sponge from the genus Grantia.Look at them as you answer the following:

• 1. If you cut this animal open, what is the name of the cavity that would see inside?
• 2.  In the center photo, what is the name of the opening shown on the left side of the animal? What is its function?
• 3.  In the photo to the right, what is the name of the spike-like structures? What are their functions? What would you speculate is their chemical composition?
• 4. If you could dissect a sponge, what internal organs would you expect to see?
• 5. The animal shown in these pictures has a unique type of cell found in no other animal in the animal kingdom. What if the name of that cell? Explain how it is important in the life of the animal.

• 6. Name three types of chemicals found in the skeletons of sponges?
• 7.T/F  To have a sedentary lifestyle as a filter-feeder, this organism has tissues and well developed organs.
• 8. T/F Sponges have specified organs for gas exchange and circulation.
• 9. Where does this animal digest its food?
• A. intracellular
• B. gastrovascular cavity
• C. stomach
• D. intestine
• 10. Does this animal have intracellular or extracellular digestion?
• 11. What is the name of the skeletal elements in this animal?
• 12. T/F: This animal has true tissues.
• 13. Does this specimen have cnidocytes?
• 14. What phylum does this organism belong to?
• 15. Water movement through a sponge would follow what path?
• a. porocyte _ spongocoel _ osculum
• b. choanocyte _ mesohyl _ spongocoel
• c. colloblast _ coelom _ porocyte
• d. blastopore _ gastrovascular cavity _ protostome
• e. colloblast _coelom _ porocyte.
• 16. T/F Muscles are well developed in sponges.
• 17. T/F Sponges have well developed internal circulatory systems.

• What causes water to move through sponges?
• What is filter feeding and how does this describe a sponge? What role do filter feeders play in aquatic ecosystems?
• Describe the pathway of water through a sponge and how food is removed from the water.
• Describe how sponges exchange respiratory gases with their environment.
• Describe how sponges get rid of waste products.
• In your own words, describe the body plan of a sponge. Make a sketch to illustrate your points.
• Discuss why sponges do not need a separate circulatory system and how the functions of the circulatory system are carried out in these animals.
• What organs did you see in the sponge?
• How are sponges uniquely different from others animals observed in lab?

Credits

Photos by Maria Oehler
Some photos from P. Crawford and Jon G. Houseman at BIODIDAC at U. of Ottawa
Diagrams from BIODIDAC at the University of Ottawa
Text and layout by W. D. Dolphin