Question

Section 28.3

1. What germ layers are present in lophotrochozoans? What type of symmetry do they have?

2. What two ciliated structures are found in lophotrochozoans? What is the purpose of each?

3. Describe flatworms in terms of their germ layer, symmetry, feeding habit, and coelom.

4. Flatworms lack a circulatory and respiratory system. How then do their cells exchange gases and obtain nutrients?

5. What are the four major classes of flatworms? How do they differ in terms of their lifestyle? Give an example of each.

6. Describe the life cycle of a tapeworm.

7. Describe the defining characteristics of rotifers. Describe them in terms of the coelom.

8. Describe the digestive system and reproductive processes of rotifers.

Answer 1

28.3

A1)

Lophotrochozoa is a superphylum in Protostomia. It is a monophyletic group which includes the phyla Annelida, Brachiopoda, Mollusca etc. the name lophotrochozoa is derived from two characteristic features exhibited by its members- the feeding structure Lophophore and the trochophore larvae in its developmental stage.

The members of Lophotrochozoa are triploblastic with three germ layers – ectoderm, mesoderm and endoderm. The embryonic mesoderm is sandwiched between ectoderm and endoderm.

A2)

Lophotrochozoans have two ciliated structures – Lophophore and two bands of cilia surrounding the trochophore larva.

Lophophore: it is the feeding structure in Lophotrochozoans. It is seen around the mouth and is composed of ciliated tentacles. Its function is filter feeding. The ridge of ciliated hollow tentacles, by their regulated motion draws in food to the mouth.

Bands of cilia on the trochophore larva: the larvae of lophotrochozoa are a small, translucent and free swimming one. It is top/pear shaped and is girdled with two bands of cilia that aids in locomotion.

A3)

Flatworms are the members of the phylum Platyhelminthes. They are flat and soft bodied invertebrates. Though some of its members are free living, 80 percent of the phylum is parasitic.

Germ layers: flatworms are triploblastic with three distinct embryonic germ layers that in turn give rise to the outer covering tissues, the internal tissues and the lining of the digestive track. The epidermal tissue is composed of a single layer of fused cells and mesodermal tissue of support and secretory cells.

Symmetry: Phylum Platyhelminthes is bilaterally symmetrical, as a longitudinal section divides the body into equal right and left halves that are mirror images of each other.

Feeding habit: Most members of the phyla are parasitic (includes flukes and tapeworms), though a few are free living. The nature of feeding differs in each member. Most members absorb nutrients through the mouth. The food reaches a gut like structure where it gets digested. Later, the nutrients are passed to the rest of the body. Flatworms lack a circulatory system.

Coelom: Flatworms are acoelomates. They do not have a body cavity like the chest cavity in humans.

A4)

Platyhelminthes lack respiratory and circulatory systems. Organisms that are less than 1mm in diameter and lack a well-developed respiratory and circulatory system resort to direct diffusion of gases into the body. In these simple organisms, every cell of the body is in close contact to the external environment. The cells are kept moist to aid fast diffusion.