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What is the primary function of the pseudocoelom in nematodes?
Which feature distinguishes Nematodes from other worm phyla?
What regulates the molting process in nematodes?
How many times do nematodes typically molt during their lifespan?
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What type of reproductive strategy is most common in nematodes?
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What is eutely in the context of nematode development?
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Which type of muscle is primarily found in nematodes?
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What is Caenorhabditis elegans best known for?
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What segment is primarily associated with chelicerae in horseshoe crabs?
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Which of the following describes the respiratory system in horseshoe crabs?
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What is a defining ecological role of horseshoe crabs?
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Which adaptive feature of arachnids helps them to capture prey?
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How does the feeding strategy of scorpions differ based on their body size?
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What distinguishes the opisthosoma of arachnids from that of horseshoe crabs?
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What characteristic of horseshoe crabs makes them unique among marine species?
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Which statement accurately describes the reproductive strategies of arachnids?
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What is the primary lifestyle of centipedes and millipedes?
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What is the function of spiracles in centipedes and millipedes?
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How are the walking legs of millipedes organized in comparison to centipedes?
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What is a notable defense mechanism employed by millipedes?
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What is the ecological role of large centipedes in the South Pacific?
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What defines the respiratory system in centipedes and millipedes?
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Which segment of centipedes is characterized by maxillipeds used for venom delivery?
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How do some species of millipedes interact in symbiosis with ants?
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What aspect of the nervous system do arthropods exhibit that contributes to their complex behavior?
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Which statement accurately describes the circulatory system of arthropods?
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What is a defining characteristic of chelicerates among arthropods?
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What reproductive strategy is most common in most arthropods?
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Which appendages are unique to chelicerates, used for feeding?
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What process must arthropods undergo to allow for an increase in body size?
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What type of visual structures do arthropods possess to detect various light spectra?
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How do arthropods control the molting process?
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Study Notes
Phylum Nematoda
- Unsegmented roundworms covered in a flexible, nonliving cuticle.
- Ecdysis: process of molting the cuticle as the worm grows.
- Sensory Organs: Paired cephalic setae and amphids on the head.
- Protostomes: bilateral and triploblastic.
- Pseudocoelom: internal organs are suspended in a fluid-filled cavity that acts as a hydrostatic skeleton.
- Belong to the Ecdysozoa clade: characterized by molting cuticles as they grow.
- Molting: regulated by the hormone ecdysone.
General Body Plan
- Typically 1-2 mm long, unsegmented worms.
- Cuticle is made of collagen.
Growth Patterns
- Growth occurs between molts, with only 4 molts throughout their lifetime.
- Eutely: growth occurs through cell enlargement after a fixed number of cell divisions.
Physiology
- Gas exchange: occurs through diffusion. No respiratory or circulatory systems.
- Digestive system: complete gut.
- Excretory system: includes gland cells and canals, opens via an excretory pore.
- Nervous system: Well-developed with a ring of nerve tissue and ganglia around the pharynx, as well as dorsal and ventral nerve cords.
- Sensory system: Includes papillae and setae for touch, anterior amphids, and posterior phasmids (likely chemoreceptors).
Reproduction
- Dioecious: separate sexes.
- Internal fertilization: sperm are ameboid (not flagellated).
- Parthenogenesis: rare form of reproduction where fertilization does not occur.
- Juvenile stages: 4 stages, each separated by a molting event.
Representative Nematodes
-
Caenorhabditis Elegans:
- model organism for neuronal development.
- first organism to have its entire genome sequenced.
Class Merostomata - Horseshoe Crabs
- Defining characteristics: Appendages on opisthosoma are flattened into ‘book gills’, final segment modified into long spike;
- Only 4 living species, remained relatively unchanged for 450 million years, with a marine habitat;
- At risk because their blood, with the blue pigment hemocyanin, is essential for biomedical research;
- Important ecologically;
- Their abdomen resembles books, referred to as "book gills";
- Gas exchange occurs in water, limited in air;
- Predator of clams and worms
Class Arachnida
- Defining characteristics: Varied opisthosoma appendages including book lungs (internal) to spinnerets;
- Mainly terrestrial;
- Examples of arachnids include spiders, scorpions, mites and ticks;
- Spiders: Solitary carnivores, usually only interact to mate;
- Scorpions: Mainly nocturnal, solitary carnivores, cannibalism common;
- Mites and ticks: Parasitism common, may live on or in association with hosts;
Appendages Modifications
- Chelicerae: Present in all, may be hollowed out for venom delivery (mainly in spiders);
- Pedipalps: Modified into claws in scorpions, blind end appendages in other groups;
- 4 pairs of walking legs;
Specialized Abdominal Appendages
- Opisthosoma often separated from prosoma by a narrow stalk (pedicel) in spiders;
- Spiders have abdominal appendages called spinnerets near the anus for silk release;
- Scorpions have modified final abdominal segment into a stinger;
Scorpions
- Cannibalistic, they can kill and eat other scorpions;
- Not immune to their own venom;
- Two methods to capture prey:
- Brute force (large body, big pedipalps);
- Venom (small body, small pedipalps).
Hardening the Exoskeleton
- Crustaceans: Add Ca2CO3 into the procuticle;
- All arthropods: Tanning;
- Protein cross-links form spontaneously in the procuticle immediately after it is secreted by epidermal cells;
Ecdysis
- To increase in body size, the entire exoskeleton must be shed at once;
- Arthropods molt many times before reaching adulthood, and some continue to molt after that;
- Old exoskeleton is degraded by enzymes and split open by allowing water or air in to "inflate" it;
- Under neural and hormonal control;
- Y-organ in the head produces ecdysteroids to trigger moulting;
- Prothoracic glands in the thorax also produce ecdysteroids;
Nervous System
- Complex brain and sensory structures in head region;
- Ventral nerve cord with metamerism (ganglia associated with body segments);
Circulation
- Open/closed hybrid system;
- Blood leaves the heart through closed vessels;
- Blood enters the heart via holes directly from the hemocoel (main body cavity);
Visual Systems
- Two levels of complexity:
- Ocelli (small cup with light sensitive surface);
- Compound eye (filled with many stacks of lenses and receptor cells organized into units called ommatidia);
- Can detect UV and polarized light;
Reproduction
- Almost exclusively sexual, some asexual, some parthenogenesis;
- Mix of internal and external fertilization;
- Many larval forms;
Subphylum CHELICERATES
- Defining characteristics: Only arthropods with no antennae, body in two sections with no distinct head, chelicerae (first pair of appendages);
- Includes spiders, scorpions, horseshoe crabs and sea spiders;
- Body divided into two regions: prosoma (head + thorax), opisthosoma (abdomen);
Appendages
- NO antennae, no mandible;
- Clawed appendages called chelicerae on either side of the mouth for food ripping and predation;
- Food manipulators with blunt ends called pedipalps;
- Walking legs (4 pairs);
Ancestors of Chelicerates
- Likely some of the first terrestrial animals;
Subphylum Mandibulata
- Includes centipedes and millipedes;
- Mainly detritovores in moist terrestrial environments;
- Some predators (centipedes);
General Features of Mandibulata
- Two tagmata = head and trunk
- Trunk is comprised of elongated thorax and reduced abdomen;
Segments of Mandibulata
- Segments 1-5 = Head:
- 1 = Antennae;
- 2 = Lost during development;
- 3 = Mandibles (jaws);
- 4 = Maxilla 1 (flappy mouth parts);
- 5 = Maxilla 2 (stringy mouth parts);
- Segments 6 onwards = Trunk:
- 6 = Maxillipeds (centipedes);
- Paired walking legs (multiple pairs, can be 10s to 100s)
Mouth Parts of Mandibulata
- Flappy primary maxillae form the lower lip;
- Mandibles/jaws are inside the mouth;
Respiration of Mandibulata
- Openings to the outside = spiracles;
- Connected to the inside via tracheal system that delivers O2 to the tissues;
Excretion of Mandibulata
- Malpighian tubules: tubules extending from the back of the gut;
- Absorb solutes and wastes from hemolymph;
- Passes solid nitrogenous waste to the gut for excretion;
Class Chilopoda - Centipedes
- One pair of walking legs per segment, spiracles on the top or side of the body;
- All venomous;
- Predatory;
Appendages Modifications of Centipedes
- 1st pair trunk/thoracic appendages (maxillipeds) modified for venom delivery;
Important Predators of Centipedes
- Large centipede in the South Pacific feeds on seabird chicks;
- Important for nutrient cycling to transfer "marine" nutrients to a terrestrial ecosystem;
Class Diplopoda - Millipedes
- Two pairs of walking legs per segment, spiracles on the bottom of the body;
- Produce noxious chemicals as a defense;
- Detritovores;
Appendage Modifications of Millipedes
- Segments of the trunk are fused together in pairs, resulting in two sets of walking legs per trunk segments;
Defense Mechanism of Millipedes
- Unlike centipedes, millipedes can't bite and are slow moving;
- Option 1 = defense coil;
- Option 2 = release noxious chemicals through holes along the side of their bodies (including cyanide);
MYRMECOPHILY
- Some species of millipedes live in close association with ants (symbiotic and commensal);
- Used by the ants to clear nest space in some cases;
- Sometimes this can turn to a toxic relationship, and if ants are too hungry, they can eat them;
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Description
Explore the fascinating world of Phylum Nematoda, unsegmented roundworms that display unique growth and physiological characteristics. This quiz covers their body plan, molting process, and essential traits like eutely and gas exchange. Test your knowledge on these intriguing creatures of the Ecdysozoa clade.
