Deck 12: Smaller Ecdysozoans

Question 1

The comments of N. A. Cobb quoted in the opening essay for this chapter make clear just how successful are nematodes. Do any characteristics of nematodes explain their much greater abundance than the other ecdysozoan phyla covered in this chapter?

Answer 1

The nematodes are the most diverse phylum after the arthropods, they can thrive in almost all the habitats, including as parasites in all types of animals and plants. Though only about 80000 species of the phylum are being demonstrated, but it is an estimation that there can be about millions of the species present. Ecologically they can be differentiated into parasitic and free living forms.
Some of the features of phylum nematodes, which describe their greater abundance in comparison to other ecdysozoan phyla, are as follows:
1. The presence of the long hollow tube, inside which another tube is present, they are round in cross section, which helps them to control their body fluids under excess pressure.
2. To maintain this excess pressure nematodes have elastic, flexible, and extremely tough cuticle. This cuticle constitutes upto 9 layers of proteinaceous fibers. Despite of its complications, the cuticle is permeable to both gases and water, thus respiration takes place through it.
3. The pharynx of nematodes is a well-organized pump, which forces food into the intestines. It plays an important role in sucking food into the mouth. Digestion is brisk and faeces are removed under pressure.
4. They have a well-developed nervous system, and the excretion of metabolic wastes takes place through two simple tubules or ducts, containing no flame cells or nephridia.
5. The process of fertilization takes place inside the host, which helps females to lay eggs for a long time period, for example, female Ascaris lumbricoides, lay her eggs at the frequency of 200000 in a day inside the host.
6. Nematodes are found in an enormous variety of habitats, due to their greater survival skills, they are present both as free living as well as in parasitic forms.

Question 2

A skeleton is a supportive structure. Explain how a hydrostatic skeleton supports an animal.

Answer 2

A hydroskeleton or hydrostatic skeleton is a composition present in various soft bodied animals comprising of the coelom, a fluid filled cavity, which is enveloped by muscles. The action and pressure of the fluid of the enclosing longitudinal, circular, or the helical muscles are used to modify an organism's shape and generate movement.
The hydrostatic skeleton is used to generate movement, such as swimming and burrowing. They preferably expand and contract the segments of their body, along their length. Some of the examples of the soft bodied organisms are earthworms and sea anemones. They play an important part in the locomotion of organisms.
Application of the pseudocoelom as a hydrostatic skeleton is a distinct characteristic of nematodes. The fluid filled pseudocoelom, within which lie the internal organs, comprises a hydrostatic skeleton, which provide support by conducting the force of muscle contraction to the noncompressible and enclosed fluid.
In nematodes, the outer body layer comprises of a non-cellular and a comparatively thick cuticle, which is secreted by the hypodermis, an underlying epidermis. This cuticle serves to contain the high hydrostatic pressure, which is applied by the fluid present within the pseudocoelom.

Question 3

What feature of body-wall muscles in nematodes requires a high hydrostatic pressure in the pseudocoelomic fluid for efficient function?

Answer 3

The feature of body-wall muscles in nematodes which requires a high hydrostatic pressure in the pseudocoelomic fluid for efficient function is movement.
Movement is possible by the action of both the longitudinal muscles and the circular muscles. These two muscles work antagonistically with the help of hydrostatic pseudocoelomic pressure to produce movement.

Question 4

Explain how high pseudocoelomic pressure affects feeding and defecation in nematodes. How could ameboid sperm be an adaptation to high hydrostatic pressure in the pseudocoelom?

Question 5

Explain the interaction of cuticle, body-wall muscles, and pseudocoelomic fluid in the locomotion of nematodes.

Question 6

Outline the life cycle of each of the following: Ascaris lumbricoides, hookworm, Enterobius vermicularis, Trichinella spiralis, Wuchereria bancrofti.

Question 7

Where in a human body are adults of each species in question 5 found?

Question 8

Where are juveniles and adults of nematomorphs found?

Question 9

What are the approximate lengths of loriciferans, priapulids, and kinorhynchs? Where are they found?

Question 10

Give the main distinguishing characteristics of Onychophora and Tardigrada.

Question 11

What do the members of each of the aforementioned groups eat, and where do they live?

Question 12

Some investigators regard Onychophora as a "missing link" between Annelida and Arthropoda. Give evidence for and against this hypothesis.

Question 13

What is the survival value of cryptobiosis in tardigrades?

Question 14

How is a hemocoel different from a true coelom?

Question 15

In what sense is a hemocoel part of the circulatory system?

Question 16

Describe the two major protostome clades, and give a defining feature for each.