(Campbell 6th Ed. 654; 7th Ed. 648-649)
Phylum Nematoda
(Campbell 6th Ed. 661-662, 7th Ed. 655-666)
These are the so-called round worms and are unsegmented. Representatives of this group may be free-living, animal parasites or plant parasites. Nematodes cause such important human diseases as ascariasis, elephantiasis, hookworm and pinworm, to mention a few. Nematodes are also important in causing diseases in various wild and domestic animals and plants.
Examine the cross-sections of Ascaris (Male
and Female)
Examine
the preserved Ascaris from your lab
Note the general body form as well as the sexual dimorphism between males and females. Most nematodes have this same basic body plan despite the variety of habitats they occupy. Note the arrangement of the three body layers, ectoderm, endoderm and mesoderm. A special body cavity known as the pseudocoelom exists between the endoderm and mesoderm.
Examine a fresh mount of the vinegar eel Turbatrix
An example of an interesting as well as important nematode is Parelaphostrongylus tenuis. The adult of this worm is found on the covering of the brain for that reason it is commonly referred to as the meningeal worm. In Manitoba, this parasite is normally found in deer and moose. It is a relatively innocuous parasite in deer, but in moose, the worm is found within the nervous tissue of the brain rather than on the covering of the brain. Here it causes serious damage often leading to the death of the infected animal. P. tenuis is a relatively common parasite in the southeastern region of Manitoba, and according to some reports, the parasite is extending its range in a westerly direction.
Phylum Rotifera
(Campbell 6th Ed. 654; 7th Ed. 648-649)
The name Rotifera is derived from the ciliated crown which gives the impression of wheels rotating. There are approximately 2,000 species of rotifers, most of which are found in freshwater with some marine, terrestrial and parasitic representatives also being found.
Reproduction in rotifers is of considerable interest. Many of the species in this area have females which produce two separate egg types. When conditions are optimal, the females produce so-called amictic eggs which are thin-shelled and diploid. These eggs develop via parthenogenesis (without fertilization) into more amictic females. Thus, under suitable conditions the rotifer population consists entirely of so-called amictic females which reproduce by parthenogenesis. If conditions change (i.e. seasonal pond dried up, winter freezes a lake, etc.) some of the amictic eggs develop into mictic females which produce thin-shelled, dormant eggs which will not develop until conditions again become suitable. Once conditions return to optimal, the egg hatches into a diploid amictic female and the cycle begins again.
Click to view a Rotifer fresh mount
Phylum Acanthocephala (thorny- headed worms)
These animals are all parasites, with the adults living in the intestine of their hosts. They attach to the gut of their host via a spiny proboscis. They are highly modified parasites and completely lack a digestive tract, in that way resembling the cestodes.
Click here to examine a preserved specimen
