Question

Section 28:3

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

2. Describe the life cycle of a tapeworm. (Also see Fig. 28.16).

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

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

Answer 1

1.

Turbellaria	Monogenea	Trematoda	Cestoda
These are the free-living kinds which are mostly found in the marine environment but at times also in the freshwater or moist terrestrial environments.	These are ectoparasites that mostly found the skin, gills, or fins of the fish.	These are internal parasites that are found in mollusks and vertebrates.	Mostly found in humans.
These are not parasitic. They make use of their ventral epidermis for locomotion.	They attach to fish to allow the maturation of their free-swimming larva into its parasitic adult form.	These have both sexual reproduction that occurs in mollusks which are the primary host and asexual reproduction which occurs in the secondary hosts like humans.	They reside in the intestine of the host organism by attaching itself using the scolex and derive its nutrition from the food passing through the intestine.
Eg. Pseudobiceros bedfordi- Bedford’s Flatworm	Eg. Dactylogyrus vastator	Eg. Clonorchis sinensis- Liver fluke	Eg. Taenia solium- pork tapeworm

2.

3.

>> They have a ciliated ‘corona’ in the head region that is used for locomotion and feeding. This ciliated corona helps to create small currents in the water that draws small planktonic foods to the rotifers.
>> They have a muscular pharynx called the ‘mastax’.
>> They have one to four toes secrete a sticky substance from the pedal glands that enables attachment.

Rotifers are pseudo-coelomates. Their body cavity is partially lined by the mesoderm. They have the entire digestive system including the mouth and an-us along with specialized organ systems. Their body cavity also acts as their hydrostatic skeleton.

4.

Digestive System of Rotifers:

They basically derive nutrition from Protozoa, and the rest of the matter is cleared by ciliary action. Rotifers are considered omnivores that consume dead and decaying matter including unicellular algae and small phytoplanktons.

• The opening and closing of the mouth are facilitated by the Sphincter and dilator muscles. The muscular structure that has hard, chitinous jaws called trophi is called the mastax. This trophi has seven pieces that include pairs of rami, unci, manubria, a median fulcrum, etc.
• The incus region is composed of the fulcrum and rami whereas the malleus is made of the unci and manubria. The manubria have a pointed posterior end called the cauda.
• From the mastax, we can see a short and narrow esophagus that leads into a ciliated sac or tube called the stomach. From the stomach, it leads to the syncytial intestine that has thin walls.
• The intestine receives the cloaca as protonephridia and oviducts that are present close to the an-us. Dilator muscles are contained in the an-us.

Reproductive System of Rotifers:

• Some rotifers undergo parthenogenesis where the mother produces its offspring from unfertilized eggs in an asexual manner.
• Some rotifers exhibit sexual dimorphism where they produce two different kinds of eggs by the process of parthenogenesis. One type of egg leads to the formation of ‘degenerate’ males that do not have a digestive system. Another kind leads to the formation of females.
• In this case, the males continue to live until the sperms can fertilize the eggs of the females leading to the formation of resistant fertilized eggs that can survive even if the water source in the surrounding environment dries up. They are released in water for hatching. In case the temperatures are too high outside, they simply stay attached to the posterior end of the rotifer.