Another Way to Categorize Animals
In an ongoing effort to understand the relationships between living organisms, scientists attempt to identify basic similarities that connect animals groups. There are many ways to organize animals based on body structure and you have already been introduced to several of these, i.e.: body shape (radial vs. bilateral); level of organization (cell vs. tissue vs. organ system); and the presence of distinct tissue layers (ectoderm, endoderm and mesoderm). The nematodes and rotifers (and several phyla we will not discuss) present yet another way to examine animal relationships. None of the animals we have studied up to this point have had a true body cavity, or coelom. The Cnidaria and Platyhelminthes are referred to as acoelomate for this reason. (Note: The Porifera are simply left out of this particular category because the cell level of organization makes them so different from all other animal phyla.) Members of phyla Nematoda and Rotifera are identified as pseudocoelomates because organs are located within a fluid filled space or body cavity. This body cavity is not lined with mesoderm tissue and so is differentiated from the so-called true coelom found in the rest of the phyla we will examine this quarter. You will observe the pseudocoel in the cross-section of Ascaris. Remember that this is a characteristic of all members of Phylum Nematoda and Phylum Rotifera.
Both phyla also exhibit a complete digestive system with mouth, intestine and anus. Specialized cells within this system allow for ingested food to be processed along a one-way passage, with unabsorbed matter finally eliminated out the anus rather than back out the oral opening as in the cnidarians. A cuticle, secreted by the epidermis, is an effective protection against harmful external substances and also serves as an exoskeleton, providing for muscle attachment.
A rather amazing characteristic of these phyla is the tendency toward
cell
constancy, in which the organs, and often the entire animal, are composed
of a precise and relatively small number of cells. (Just as an example,
the central nervous system of Ascaris and many other nematodes is
made up of 162 cells.)
A Closer Look at Phylum Nematoda
After reading Chapter 13 in Buschbaum, you should appreciate nematodes
as highly successful organisms. As a group they are incredibly cosmopolitan,
found in nearly every type of habitat and location on earth. Even so, it
is considered that the majority of nematode species are still undescribed
and unknown to science!
Use the diagrams and photos in Buschbaum to help identify the internal
and external structures you observe in the prepared slides of Ascaris
and Trichinella. These can be found in the slide box at your table
or set up on demonstration. Check the objectives list for the structures
that need to be drawn and labeled for study.
Live Vinegar Eels
On one of the front tables is a culture of vinegar eels. Observe them through the stereoscope and note the swimming movements of these animals. This whiplike motion is typical of nematodes and a function of the longitudinal muscles and lack of circular muscles characteristic of this phylum. Notice the vinegar eels are different sizes. The smallest ones are juveniles. This species is ovoviviparous, that is, the fertilized eggs develop within the female, but are dependent upon nutrition from yolk material within the egg. The young are born live.
Prepare a wet mount with one drop of the vinegar and add one drop of
methyl cellulose and put a coverslip in place. Get a compound scope from
the cabinet. Once you have found a worm using the lowest power objective,
focus it on a higher power. The worms will be mostly transparent and you
should be able to identify the other structures that characterize this
phylum. Though you will not be tested on the internal anatomy of the vinegar
eel, the diagrams at your table will help you to know what you are looking
at. Can you determine whether the worm you are observing is a male or a
female?
Phylum Rotifera
A second type of pseudocoelomate is available for study in lab. As explained in Buschbaum, Phylum Rotifera is a relatively small, but very interesting group. All rotifers are aquatic and microscopic. In addition to the characteristics described on the front side of this page, rotifers are known for a peculiar life history in which males are rare and even unknown in some species! For most of the year, female rotifers produce diploid eggs that develop without need of fertilization, a process known, as parthenogenesis. All the offspring produced this way are female. During a certain time of the year, some females will produce haploid eggs and some of these will hatch into male rotifers. Then, both males and females produce haploid gametes. Mating occurs and the subsequent fertilization of the haploid eggs by the haploid sperm results in all female (and diploid) offspring.
Examine the culture of live rotifers set up under a stereoscope at the front table. Most of the animals will be feeding while attached to sediment or to the bottom of the container. As you are observing, you will notice other rotifers swim by, aided by the action of the cilia located near the mouth. Focus on the bottom of the container and you will see a second type of locomotion in which the animal moves along the substrate much like an inchworm.
Use the dropper as you look through the microscope and remove one individual
from the container. Prepare a wet mount using a clean slide and a coverslip
and then locate the animal using the lowest power of the compound scope
at your seat. Focus under high power adjusting the scope fight as needed.
Draw the rotifer using the diagram on p.235 to help you identify and label
the external and internal structures you are responsible for.
KINGDOM ANIMALIA: Phylum Nematoda