(entry in progress)
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Thursday, March 29, 2012
Wednesday, March 28, 2012
Lecture - Phylum Mollusca
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| Generalized mollusc from Biodidac |
Today we continued our discussion of molluscs by explaining general features using the generalized mollusc as a base line to later on explain the adaptive radiation of the various molluscan groups.
We had a brief discussion of the head-foot portion, with emphasis on the head portion and a structure that is present in most mollusc groups, the radula. Then we focused our attention on the foot, its roles, and some of its variants in different groups.
We also talked about the mantle and its role in secreting the three-layered shell (periostracum, prismatic layer, nacreous layer), and the importance of it lining the mantle cavity, where the gills, in the form of ctenidia, are found.
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Friday, March 23, 2012
Lecture - Phyla Nematoda and Mollusca
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| Parasitic nematodes Top: A hookworm and SEM of the anterior end of one Middle: A pinworm and a Guinea worm Bottom: A large human roundworm and a an intestine infested with them |
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Today we finished the chapter on nematodes, by discussing aspects of their reproduction, nervous system, and details about their lifestyles (parasitic and free living).
Among the parasitic nematodes we looked into the anatomy and life cycles of hookworms, pinworms, large human roundworm, filarial nematodes, and Guinea worms.
We closed the lecture with a brief discussion of Nematoda en its (still debated) placement in the Ecdysozoa clade within the greater metazoan phylogeny.
We then started our discussion on the phylum Mollusca, by briefly discussing which groups comprise the phylum, how ancient this lineage is, and the great diversity found within it. We reviewed some of the more general characteristics (eucoelomate, unsegmented, protostomes) and introduced the concept of the Hypothetical Ancestral Mollusc (HAM), also known as the generalized mollusc or the archetype mollusc
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Thursday, March 22, 2012
Lecture - Phyla Rotifera and Nematoda
WEDNESDAY, MARCH 21, 2012
Today we started covering a couple of examples of pseudocoelomate phyla, Rotifera and Nematoda.
Rotifera, the "wheel bearers", are an interesting phylum of microscopic or nearly microscopic, mostly freshwater organisms, typically with a cephalic structure called crown, bearing a series of cilia that beat metachronally, giving the impression that a wheel is spinning. Such behavior is used by most members of this phylum to feed and swim.
We studied their main external anatomical features, feeding and swimming/"walking" behavior, their mode of osmoregulation, and their nervous system.
Then we had an overview of the main groups, Digonatans (Seisonids and Bdelloids) and Monongonontans.
When discussing the bdelloids we emphasized the fact that they reproduce parthenogenetically and no males of this group have ever been observed.
They can also undergo cryptobiosis when environmental conditions change drastically. They go into a desiccate dormant state, in some groups a cyst, that can be rehydrated months to years later, depending on the group. Cysts can be exposed to extreme conditions and still be able to rehydrate. Examples are exposure to -200º C for a few days, or exposure to nearly absolute zero temperatures (0.008º K) for a few seconds.
Monogonontan rotifer populations are composed exclusively by females, but males have been observed. When environmental conditions become stressful eggs are produced by meiosis (instead of the normal mitotic process), allowing for the birth of haploid males and females. These reproduce producing diploid eggs that are very resistant to adverse conditions, and will hatch when the stress has receded.
Nematodes, unsegmented "roundworms" are the most ubiquitous and abundant animals on earth. Insects may be the most diverse (so far), but in terms of number, nematodes take the prize. They are so abundant (free living and parasitic) that it is said that "if everything on Earth were to disappear except the nematodes, the outlines of everything would still be visible: the mountains, lakes and oceans, the plants and the animals would all be outlined by the nematodes living in every habitat." (I do not know the original source of such statement.
We discussed a cross section of their internal anatomy, their digestive and nervous systems.
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Today we started covering a couple of examples of pseudocoelomate phyla, Rotifera and Nematoda.
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| Left: Philodina gregaria (Rotifera: Bdelloidea - Micrographia) Right: Stephanoceros fimbriatus (Rotifera: Monogononta - Water Bear web base) |
Rotifera, the "wheel bearers", are an interesting phylum of microscopic or nearly microscopic, mostly freshwater organisms, typically with a cephalic structure called crown, bearing a series of cilia that beat metachronally, giving the impression that a wheel is spinning. Such behavior is used by most members of this phylum to feed and swim.
We studied their main external anatomical features, feeding and swimming/"walking" behavior, their mode of osmoregulation, and their nervous system.
Then we had an overview of the main groups, Digonatans (Seisonids and Bdelloids) and Monongonontans.
When discussing the bdelloids we emphasized the fact that they reproduce parthenogenetically and no males of this group have ever been observed.
They can also undergo cryptobiosis when environmental conditions change drastically. They go into a desiccate dormant state, in some groups a cyst, that can be rehydrated months to years later, depending on the group. Cysts can be exposed to extreme conditions and still be able to rehydrate. Examples are exposure to -200º C for a few days, or exposure to nearly absolute zero temperatures (0.008º K) for a few seconds.
Monogonontan rotifer populations are composed exclusively by females, but males have been observed. When environmental conditions become stressful eggs are produced by meiosis (instead of the normal mitotic process), allowing for the birth of haploid males and females. These reproduce producing diploid eggs that are very resistant to adverse conditions, and will hatch when the stress has receded.
A bdelloid rotifer feeding
A monogonont rotifer feeding
Nematodes, unsegmented "roundworms" are the most ubiquitous and abundant animals on earth. Insects may be the most diverse (so far), but in terms of number, nematodes take the prize. They are so abundant (free living and parasitic) that it is said that "if everything on Earth were to disappear except the nematodes, the outlines of everything would still be visible: the mountains, lakes and oceans, the plants and the animals would all be outlined by the nematodes living in every habitat." (I do not know the original source of such statement.
We discussed a cross section of their internal anatomy, their digestive and nervous systems.
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| A (morphologically) typical nematode Nova Scotia nematodes |
Thursday, March 15, 2012
Lecture - Phyla Platyhelminthes and Nemertea
WEDNESDAY, MARCH 14, 2012
PLATYHELMINTHES
We briefly covered some of the main features of Monogenea, prominent fish ectoparasites, focusing on their external anatomy, which includes the presence of a complex structure at the posterior end, an opisthaptor, and an anterior adhesion organ, a prohaptor.
We then moved on to discuss the flukes, Trematoda. Some of their main characteristics are:
The stages that can enter a host are "equipped" with glands that produce enzymes that disrupt the host's tissues. One of the most interesting characteristics of trematodes is their capability to detect the correct host, and ignore individuals that cannot host them.
NEMERTEA - Ribbon worms
The nemerteans, or "ribbon worms", are a phylum of unsegmented, dorsoventrally flat, mostly marine, organisms, that have a muscular, evertible proboscis independent from the digestive system. In some cases there is only one opening shared by the mouth and the proboscis, but still the cavity containing the proboscis, the rhynchocoel, is separate from the digestive tract.
We discussed locomotion, and the nervous, excretory, digestive and circulatory systems.
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PLATYHELMINTHES
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| An SEM and a diagram of a monogenean platyhelminth |
We briefly covered some of the main features of Monogenea, prominent fish ectoparasites, focusing on their external anatomy, which includes the presence of a complex structure at the posterior end, an opisthaptor, and an anterior adhesion organ, a prohaptor.
We then moved on to discuss the flukes, Trematoda. Some of their main characteristics are:
- They are all parasitic
- They use at least two hosts in their life cycle
- The intermediate host is usually a snail
- Final hosts are vertebrates
- Most of them are hermaphrodites
The stages that can enter a host are "equipped" with glands that produce enzymes that disrupt the host's tissues. One of the most interesting characteristics of trematodes is their capability to detect the correct host, and ignore individuals that cannot host them.
NEMERTEA - Ribbon worms
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| Nemerteans in natural habitat and with the proboscis everted |
The nemerteans, or "ribbon worms", are a phylum of unsegmented, dorsoventrally flat, mostly marine, organisms, that have a muscular, evertible proboscis independent from the digestive system. In some cases there is only one opening shared by the mouth and the proboscis, but still the cavity containing the proboscis, the rhynchocoel, is separate from the digestive tract.
We discussed locomotion, and the nervous, excretory, digestive and circulatory systems.
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Saturday, March 3, 2012
Lecture - Phylum Platyhelminthes
FRIDAY, MARCH 2, 2012
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(entry on progress)
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(entry on progress)
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Thursday, March 1, 2012
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