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IB104 - Lecture 4 - Worms Reading - Chapter 25.2, 6, 10, 11 1. The rest of the animal phyla are all bilaterally symmetrical, and are clearly a monophyletic clade. They are therefore referred to as the Bilateria, which might be considered a “subkingdom”. All Bilateria also have three body layers or are triploblastic, with the mesoderm forming between ectoderm and endoderm, yielding muscles and eventually bones. An older, “progressive”, view of animal phylogeny in which the presence of a body cavity lined with mesoderm, known as a coelom, was considered to be a defining characteristic of a large group of animals including ourselves (Chordates). Chordates Echinoderms Arthropods Annelids Mollusks Coelomate Ancestry Rotifers Roundworms Bilateral Ancestry Flatworms Radial Ancestry Multicelled Ancestry Bilateral symmetry Radial symmetry Arthropods mouth coelom segmented body Annelids Mollusks head Classical animal phylogeny: based on coelom chordates echinoderms arthropods tardigrades annelids mollusks no coelom unsegmented body Flatworms mouth flatworms Acoelomate animals Animals with a 3-layer embryo cnidarians saclike gut Protostomes rotifers Pseudocoelomate animals roundworms complete digestive system radial symmetry, no cephalization Deuterostomes Coelomate animals false coelom bilateral symmetry, cephalization Placozoa Sponges Single-celled, Protista-like ancestors Roundworms anus Cnidarians Cnidarians sponges placozoans Animals with tissues Animals Fig. 25-7a, p. 407 1 Protostomes (the phyla today and Wednesday): literally “first mouth”, because the mouth forms from the blastopore which is formed during gastrulation, while the anus forms from a secondary invagination at the other end of the body, which then connects with the rest of the gut, developing from the archenteron (the space enclosed by endoderm). Deuterostomes (Echinoderms and Chordates including us - Friday): literally “second mouth”, because the blastopore forms the anus, while the mouth forms from a secondary invagination. Alternative: Phylogeny based on DNA and proteins Deuterostomes chordates echinoderms arthropods tardigrades Ecdysozoa roundworms Protostomes rotifers mollusks Lophotrochozoa annelids flatworms Animals with a 3-layer embryo cnidarians sponges placozoans Animals with tissues Animals blastopore Fig. 25-7b, p. 407 2. Phylum Platyhelminthes: The ±15,000 flatworms are so-named because they are flat. They have a clear layer of mesoderm between the ectoderm and endoderm, and this layer of cells gives rise to various internal organs including an excretory system and muscles. They have a well-developed nervous system, including a concentration of sensory systems for touch, smell, and even primitive sight, at the anterior end this is called cephalization. This is part of being a mobile animal that navigates across surfaces and encounters things with it’s head first. Platyhelminthes have a blind gut with no anus, and they also have no body cavity and no circulatory system, hence breathe through their body wall, which is only possible because they are flat. In the absence of a coelom-like body cavity they are called acoelomates. 3. All of these features are most obvious in one of the three major classes, the Turbellaria or planarians. These are free-living animals, mostly marine but some freshwater, that eat small animals and decaying flesh. They have a extrudable pharynx that opens into the blind gut. brain nerve cord ovary branching gut pharynx (protruded) protonephridia testes oviduct genital pore penis Note - no circulatory system and no coelom – ancestral or secondarily lost? 2 Land planarian 5. The second class are the Trematoda or flukes. These are all parasitic worms, commonly with complicated lifecycles, some of which infect humans. The most serious are the genus Schistosoma, which cause schistosomiasis or bilharzia. These tiny worms infect and grow in freshwater snails, and then can enter human skin, travel to the intestine and bladder to reproduce in large numbers, and then complete the lifecycle by exiting via urine or feces back into freshwater. Chronic infection results in damage to the liver, intestines, and bladder, largely through the body’s own defensive reactions to the Schistosoma eggs. These are major parasites in the developing world (estimated at 200 million cases per year), and generally only affect “adventure” travelers from the US. There are major efforts to control both snails and worms, and a new drug called praziquantel is remarkably effective against the worms, with a single dose controlling them for five years. A wonderful topic for the Animal Planet show “Monsters Inside Us”. Flukes do occur in the USA, and cause “swimmer’s itch”, but it is a different genus of worms and not nearly as serious as schistosomiasis. 4. Planaria reproduce sexually, but are also capable of asexual reproduction by literally splitting in two from front to back, and then regenerating both symmetrical sides. This remarkable capacity for regeneration is being exploited in research labs where scientists like Phillip Newmark in our Cell and Developmental Biology department are studying the genes and proteins involved (handout). We would like to understand why we humans are only capable of regenerating certain tissues and organs. The answer almost certainly involves at least in part the remarkable properties of stem cells, which are capable of regenerating both themselves and other more specialized cells. Demonstrate by cutting flatworms in half, but one can actually cut them in tens of pieces and each is capable of regenerating into a whole worm. This implies that every part of the flatworm body “knows” what and where it should be, and that information can be used to regenerate the neighboring missing cells and tissues. It turns out that this is achieved by specialized “adult stem cells” called neoblasts that regenerate the missing parts - would that we could do the same for all of our body. Schistosoma lifecycle. 3 6. The third class are the Cestoda or tapeworms. These are primarily gut parasites of vertebrates with extremely modified and simplified morphology. A barbed head attaches to the gut lining, and the rest of the body is repeated reproductive segments, which exit with faeces. 7. Phylum Nemertea: These are long flat colorful ribbonworms. Entirely marine and about 1000 species known. They prey on other animals using a unique proboscis. In contrast to the flatworms they have a complete gut with an anus at the other end. As before, the mouth forms from the blastopore created at gastrulation, while the anus forms from a secondary invagination near the posterior end. © Copyright Glencoe-McGraw Hill, 2000 8. Phylum Nematoda: These are called roundworms because they have a body cavity, so in cross-section they are round. There are >20,000 species, and these are a major under-appreciated fauna in most environments. A body cavity forms between the ectoderm and endoderm and contains tissues formed from mesoderm, including a circulatory system. This pseudocoelom (so-called because it is not considered a true coelom, which must be lined with mesoderm) is kept open by hydrostatic pressure against the outer hard cuticle. Alternating layers of longitudinal and circular muscles allow sinusoidal movement. 9. Filariasis is a parasitic disease caused by nematodes of two species, Brugia malayi and Wucheria bancrofti. These tropical diseases are transmitted by mosquitoes. Like flatworms, it is difficult to develop drugs against these animal parasites, as they have similar physiology, mediated by similar genes and proteins, to us. The worms block lymph nodes, and if untreated lead to massive swelling, especially of the legs, called elephantiasis. 4 10. Caenorhabditis elegans is a free-living, tiny, 1mm-long, soil nematode that has become a model organism in molecular biology. Each of its 959 cells is fully described and located, all cell divisions, including cell death, are known, and all connections between all nerves are known. All of its ~100 million base pair DNA have been sequenced (indeed, the first animal genome completely deciphered in 1998) to uncover all ~20,000 genes in its genome encoding all the different proteins needed to make an ecologically functioning animal, including ~1000 chemoreceptors that I study (5% of all its genes/proteins). 11. Phylum Rotifera: Rotifers don’t look like worms, but their level of complexity is similar, including a pseudocoelom body cavity. They attach to a substrate and filter bacteria, protists, and algae using beating cilia. We are not sure where they fit in phylogenetically. 12. Phylum Tardigrada: The tardigrades (meaning “slow walkers”) are also known as “water bears”. They are tiny animals less than 2 mm in length, with perhaps 1000 species, which live primarily in moist places like on mosses. Their most distinctive feature is that they are able to survive complete desiccation (as well as various other extreme insults, some even suggest outer space!). Just add water and they come back to life. They do this by packing their cells with sugars that protect them. 5