Acoelomates: Phylum Platyhelminthes and Nemertea - Biosciweb.net

Acoelomates: Phylum Platyhelminthes and Nemertea and
Pseudocoelomates: Phyla Nematoda and Rotifera & Parasitism 6.4
Lab #6 - Biological Sciences 102 – Animal Biology
Classification & Adaptations of Pseudocoelomate Animals
Phylum Nematoda (roundworms)
More than 25,000 species. Roundworms. Nematodes are an extensive group with worldwide
distribution. They include terrestrial, freshwater, marine, and parasitic forms. They are
elongated roundworms covered with a flexible, nonliving cuticle. Circular muscles are lacking
in the body wall and, in Ascaris, longitudinal muscles are arranged in four groups separated by
epidermal cords (some nematodes have six or eight groups of longitudinal muscles). Cilia are
completely lacking. Nematodes are found free-living in almost every conceivable habitat from
arid deserts to lake bottoms, rivers, polar seas. Nematodes - both parasitic and freeliving - are
incredibly abundant. A handful of good garden soil contains thousands of nematodes. Some
50 different species of nematodes occur in humans, most of them nonpathogenic. Some
nematodes are plant parasites feeding on plant sap, especially roots.
Body Plan Features & Characteristics of Members of the Phylum Nematoda
(some of these traits are seen in other animal phyla):
1. pseudocoelomate
2. bilateral symmetry
3. triploblastic structure (endoderm, mesoderm and ectoderm)
4. psuedocoelomate = possess a pseudocoelom
5. some degree of cephalization = head with sensory organs
6. some organ systems (formed of true tissues) are present
7. vermiform (worm-like) body shape
8. complete digestive tract with three angled (longitudinal, circular and oblique)
muscular pharynx, intestine lacking muscular walls, short rectum and anus
9. excretion and osmoregulation through the cuticle and by excretory canals
10. lack both cilia and flame cells/bulbs
11. body wall lined with cuticle which retains body shape
12. longitudinal muscles only, no circular muscles in body wall; body length or
diameter does not change in movement
13. hydrostatic skeleton (as muscles push against parenchyma and tissue fluids)
14. dioecious; sexes separate with females larger than males; one or two tubular gonads;
copulatory spicules present in males
Phylum Rotifera (rotifers)
There are about 2000 species of rotifers. The name "rotifer" is derived from the Latin word
meaning "wheel-bearer"; making reference to the crown of cilia around the mouth of the rotifer.
The rapid movement of the cilia in some species makes them appear to whirl like a wheel.
Rotifers can be found in many freshwater environments and in moist soil. The habitat of
rotifers may include still water environments, such as lake bottoms, as well as flowing water
environments, such as rivers or streams. Rotifers are also commonly found on mosses and
lichens growing on tree trunks and rocks, in rain gutters and puddles, in soil or leaf litter, on
mushrooms growing near dead trees, in tanks of sewage treatment plants, and even on
freshwater crustaceans and aquatic insect larvae. Most species of rotifers are about 200 to 500
micrometers long. However a few species, such as Rotaria neptunia may be longer than a
millimeter. Rotifers are thus multicellular creatures who make their living at the scale of
unicellular protists. Characteristic features of rotifers include the ciliated corona ("wheel
organ") and the mastax. The corona is located anteriorly and functions in locomotion and food
gathering. The corona is modified extensively in some species. The mastax is a muscular
pharynx containing a complex set of hard jaws or trophi and is found in all rotifers. These
characteristics of these structures have been used extensively in classifying rotifers.