Cnidaria -Workbook Questions

What other types of symmetry can animals have?  (p. 20) 

The following symmetries can be found in animals: asymmetric, spherical, radial, biradial, and bilateral. 

What’s the difference between a mesoglea and a mesenchyme found between the two layers of a diploblast?  (p. 20)  

Both terms refer to the gelatinous matrix found between the ectoderm and endoderm layers of cells. If there aren’t any, or only a few cells in the matrix, it’s referred to as mesoglea. If there are a lot of cells in the matrix, then it’s mesenchyme.   

What are some of the different roles the medusa and polyp have in the different cnidarian classes?  (p. 22)  

Each of the four different classes of cnidarians can be defined by the role that the medusa and polyp have in their life cycles. In the hydrozoans, both stages play an equally important role, with the medusa as the reproductive stage. In Anthozoa the medusa has disappeared. Corals and anemones spend their life cycle in the polyp stage, which is also reproductive. In the schyphozoan life cycle, the reproductive medusa dominates, and the polyp is only short-lived, appearing as the scyphostome. Cubozoans live their lives as medusa, and the polyp stage has disappeared. 

Why is the planula important in theories of metazoan evolution?  (p. 22) 

Early guesses on the metazoan origins looked for biological examples as evidence for how single-cell organisms became multicellular animals. One of the places biologists first looked for clues was in embryological development, ontogeny; the changes from a single-cell zygote to a hollow-balled blastula; and finally the gastrula with two cell layers of ectoderm and endoderm. The simplest blastula is a hollow ball, a single cell layer thick. Its similarities to the colonial protozoan Volvox, also a hollow ball of colonial cells, seemed to be a good clue to how multicellularity arose. There were problems with using Volvox as the model for the beginnings of multicellular animals, not the least of which is that it’s a plant and its body cells are haploid, not diploid! Blastulas also come in other forms, including a solid ball of cells; that’s what a planula looks like. Planula’s settle to the ground and differentiate into polyps with ectoderm and endoderm and a hollow cavity inside, the gut. This was thought to be similar to the gastrulation, which creates a ball of cells with the same two cell layers and a primitive gut, the archenterons, inside.    

Why is the velum important in how the hydrozoan medusa swims?  (p. 23) 

The medusa swims by contracting and expanding the umbrellar surface, pumping water in and out of the space underneath, the subumbrellar surface. As the medusa increases in diameter, water is pulled in; as it contracts, water is pushed out of the subumbrellar space. If the diameter of the opening to the space is the same when it fills and empties, the generated propulsive forces would be equal in each direction. The medusa would rise then fall the same distance -- not an effective way of moving. The velum is a ridge of tissue on the inner most edge of the subumbrellar surface. As the medusa decreases in diameter, the velum decreases the size of the opening to the subumbrellar space; as the medusa expands and fills, the velum folds back and creates a larger diameter opening. When water moves through a smaller opening it speeds up, creating a greater propulsive force when the velum decreases the diameter of the opening. The result is the jerky rise and fall, with more distance being gained on the rise.   

How many different types of polyps does the Portuguese man-of-war have?  (p. 23) 

The Portuguese man-of-war is a colonial hydrozoan and has the following types of polyps: gastrozooids, feeding polyps; gonozooids, reproductive polyps; dactylozooids, defensive polyps; and a pneumatophore, a polyp modified as the gas-filled float. 

What moves the fluid in a jellyfish’s gastrovascular cavity?  (p. 24) 

The cells that line the gastrovascular cavity have flagella; they help to keep the contents of the cavity well mixed. 

If cnidarians are diploblastic, lacking mesoderm, how can an anemone have retractor muscles?  (p. 24)  

We usually think of muscle cells as being only derived from mesoderm, and that’s why there may be some confusion. Ectodermal cells can also be contractile.  A good example is the nutritive muscular cells that line the gastrovascular cavity. These are myoepithelial cells, and in the septal walls of the anemone, they are concentrated in bands creating the retractor muscles. Not everyone agrees that this is the case, and there is some thought that all animals are triploblastic and the diploblastic condition that we see in the cnidarians is a derived, rather than ancestral, character. If that’s the case, then the muscles of the anemone may be more closely related to muscles in other animals than was originally thought.

Protozoa || Porifera || Cnidaria || Platyhelminthes || Nematoda || Annelida || Mollusca || Arthropoda
Echinodermata || Chordate Origins || Jawed Fishes || Amphibia || Mammalia