Animal Skeletons, Respiration, and Circulation - Study Notes

Animal skeletons: hydrostatic vs exoskeleton vs endoskeleton

Types of skeletons covered: hydrostatic (fluid-filled cavity), exoskeleton (external shell/rigid shell), endoskeleton (internal skeleton).
Learning focus: compare characteristics, respiration forms, circulatory system components across animals.

Hydrostatic skeletons

Definition: fluid-filled body cavity that provides support and shape; movement is produced by muscle action around the cavity.
Key examples in the slides:
- Sea anemone (Cnidaria)
- Earthworm (Annelida)
Structural features (illustrated concepts):
- Fluid-filled body cavity acts as the skeleton.
- Body wall composed of epidermis and muscle layers.
- Longitudinal muscles and circular muscles interact to produce locomotion and body shape changes.
- In earthworms, the coelom functions as the hydrostatic skeleton; muscles work against the fluid-filled cavity.
Significance: allows soft-bodied animals to move without rigid internal or external frameworks; movement relies on hydrostatic pressure and muscle coordination.

Exoskeletons

Definition: rigid outer shell that supports and protects the body; muscles attach to the inside of the skeleton.
Key examples in the slides:
- Grasshopper (Arthropod)
- Nautilus (Mollusca) with external shell
- Bivalve molluscs (e.g., clams, oysters) with external shells
Structural features:
- External protective shell or skeleton provides rigidity.
- Jointed appendages in arthropods allow movement within the constraints of the exoskeleton.
Significance: limits water loss and provides defense; requires molting/ecdysis in many groups (e.g., arthropods) to grow.

Endoskeletons

Definition: internal framework (bones/cartilage) that supports the body; muscles attach externally to the inside of the skeleton.
Key concept shown: vertebrate-type endoskeleton; presence of spinal column and discs in a vertebrate pattern.
Significance: allows large, enabled body sizes and complex organ placement; supports advanced locomotion and extensive musculature.

Hydrostatic skeleton details (related anatomy)

Spinal cord, nerve roots, vertebrae, and intervertebral discs (illustrated in vertebrate endoskeleton context) indicate a different skeletal plan (internal bone-based) than hydrostatic systems; highlights diversity of structural support across phyla.

Learning outcome alignment

Compare characteristics of different skeletal types.
Relate skeletal type to habitat and lifestyle (soft-bodied vs rigid-bodied movement).

Animal respiration systems (overview)

Spectrum of respiration forms across groups:
- None or diffusion-based surfaces (diffusion across skin or body surface).
- External and internal gills in aquatic forms.
- Specialized tracheal systems in terrestrial arthropods (trachea/tracheoles and spiracles).
- Lungs with negative pressure ventilation in many terrestrial vertebrates.
- Flow-through lungs with air sacs in birds (posterior/anterior air sacs contribute to unidirectional air flow).
Specific examples and notes from slides:
- Flatworms (Platyhelminthes): diffusion across skin; digestive cavity participates in gas exchange.
- Tube worms and fish: gills with lamellae and filaments for gas exchange.
- Aquatic salamanders: external vs internal gills (developmental and species variation).
- Terrestrial arthropods: tracheal system with spiracles; gas exchange occurs directly via tracheoles.
- Negative pressure ventilation: lungs expanded by diaphragmatic and thoracic muscle movements drawing air in (inhalation) and relaxing to expel air (exhalation).
- Flow-through lungs: birds exhibit anterior and posterior air sacs with air flow through lungs for efficient gas exchange even during rapid activity.
Key terms to remember:
- Negative pressure ventilation: air is pulled into the lungs by a decrease in thoracic pressure.
- Positive pressure ventilation: air is pushed into lungs (less common in vertebrates, more typical of amphibians under certain conditions).
- Tracheal system: network of tubes delivering air directly to tissues in terrestrial arthropods.
- Spiracle: external opening to the tracheal system.
- Gills: specialized aquatic respiratory surfaces with lamellae and filaments; can be internal or external depending on the taxon.
- Flow-through lungs: continuous water/air flow through lungs aided by air sacs (e.g., birds).

Animal circulatory systems (overview)

Open circulation
- Features: tubular heart; venous cavities drain into body cavity; blood vessels open into sinuses surrounding organs.
- Common in many invertebrates with less complex metabolic demands.
Closed circulation
- Single circulation: blood passes through two sets of vessels once per circuit; typical of many fish.
- Double circulation: blood passes through the heart twice per complete circuit (e.g., amphibians, reptiles, birds, mammals).
- Structure cues: dorsal and ventral blood vessels; capillary beds (gill/lung/capillaries); systemic capillaries.
Significance: Closed systems generally allow higher metabolic rates and more efficient oxygen transport; open systems are adequate for smaller or less active organisms.

Activity 1, Part 1: Classifications of skeletons (conceptual primer)

There are three primary skeleton types:
- Hydrostatic skeleton
- Exoskeleton
- Endoskeleton
Phylogenetic framing: identify where each type first appeared in the evolutionary history (phylogeny) of major animal groups:
- Proposed early appearances include:
- Hydrostatic: among soft-bodied groups like Cnidaria and Annelida.
- Exoskeleton: in Arthropoda (and some shell-bearing Mollusca).
- Endoskeleton: in Echinodermata and later in Chordata.
This activity builds a foundation for understanding how environmental pressures and body plans co-evolved with skeletal strategies.

Activity 1, Part 2: Mystery animals (fill-in exercise)

Goal: use the following terms to identify mystery animals by combining skeleton type, respiration, circulatory system, and feeding mode.
Given options (to select per row):
- Common names include: Tiger, Tiger flatworm, Tigerworm, Barramundi, Dragonfly, Sea anemone, Humpback whale, Falcon, Giant clam
- Phyla include: Platyhelminthes (trematodes), Arthropoda, Annelida, Chordata, Cnidaria, Mollusca
- Skeleton types: Exoskeleton, Hydrostatic, Endoskeleton
- Respiratory systems: Flow-through lungs, none/diffusion, gills, trachea, negative pressure lungs (tidal lungs)
- Circulatory systems: Closed double circulation, none, closed single circulation, open
Example mappings (illustrative, based on typical biology):
- A: Sea anemone — Phylum: Cnidaria; Skeleton: None (diffusion); Respiratory: none; Circulatory: none; Feeding: Sedentary filter feeder.
- B: Barramundi — Phylum: Chordata; Skeleton: Endoskeleton; Respiratory: Gills; Circulatory: Closed single circulation; Feeding: Predator.
- C: Dragonfly — Phylum: Arthropoda; Skeleton: Exoskeleton; Respiratory: Trachea; Circulatory: Predator (actively predatory lifestyle).
- D: Tigerworm — Phylum: Annelida; Skeleton: Hydrostatic; Respiratory: none; Circulatory: Predator/Scavenger (detritivore/detritus feeder in practice).
- E: Giant clam — Phylum: Mollusca; Skeleton: Hydrostatic (& Exoskeleton, via shell); Respiratory: Gills; Circulatory: Open; Feeding: Sedentary filter feeder.
- F: Humpback whale — Phylum: Chordata; Skeleton: Endoskeleton; Respiratory: Negative pressure lungs; Circulatory: Closed double circulation; Feeding: Filter feeder.
- G: Falcon — Phylum: Chordata; Skeleton: Endoskeleton; Respiratory: Flow-through lung; Circulatory: Closed double circulation; Feeding: Predator.
- H: Tiger — Phylum: Chordata; Skeleton: Endoskeleton; Respiratory: Negative pressure lungs (tidal) OR Flow-through lungs (depending on the sub-group, birds vs mammals); Circulatory: Closed double circulation; Feeding: Very fast predator.
- I: Tigerworm (or Tiger flatworm, depending on the row) — Phylum: Annelida or Platyhelminthes; Skeleton: Hydrostatic; Respiratory: none; Circulatory: Closed single circulation (multiple hearts in annelids); Feeding: Decomposer/Scavenger.
Takeaway: Some combinations are common and others are anatomically implausible; the exercise reinforces the link between skeleton type, respiration, circulatory design, and feeding strategy.

Activity 2: Myth busting the movies (biological reality check)

The Megalodon in The Meg (2018):
- Claimed size: about $23 ext{ m}$ long; could bisect a humpback whale (~$12 ext{ m}$) in one bite.
- Realistic assessment: not likely to bisect a whale in one bite; while Megalodon was enormous, actual bite mechanics, prey resistance, and jaw architecture do not support guaranteed mid-body dismemberment of a large whale in a single bite.
- Real-world constraint note: bite force, gape, and prey handling depend on jaw shape and tooth arrangement; extremely large sizes do not automatically equate to simple one-bite dismemberment of large prey.
- Conclusion: Hollywood exaggeration; biology imposes mechanical and ecological limits.
Tremors (1990): Graboids are giant worms with three prehensile tentacles in their throats.
- Realistic evaluation: invertebrate biology does not support this anatomy; worm bodies and feeding strategies do not align with snake-like tentacle prey capture at that scale.
- Conclusion: not biologically plausible given known invertebrate morphology.
Pirates of the Caribbean 2: Kraken destroying The Black Pearl (50 m galleon) with a 20 m-long kraken.
- Realistic evaluation: cephalopod biology shows tentacle-heavy predators, but a free-swimming 20 m long invertebrate capable of sinking a sailing ship is not supported by known anatomy or biomechanics.
- Conclusion: extreme fiction not aligned with invertebrate capabilities and physics.
Mothra (1962): Giant moth with wingspan 250 m using silk to cocoon enemies and showing curiosity/aggressiveness.
- Realistic evaluation: scaling laws (e.g., square-cube law) and insect physiology limit maximum wing span and silk production; a 250 m wingspan would require unrealistic muscle mass, energy, and exoskeletal strength.
- Conclusion: not feasible given real-world insect biology.

Conclusion: how skeletons, respiration, and circulation relate to biology

Skeleton type, respiration method, and circulatory system are interconnected and influenced by clade (phylogeny), environment, lifestyle, and body size.
Expect major patterns:
- Invertebrates with soft bodies often rely on hydrostatic skeletons or exoskeletons for protection and support.
- Larger, more active vertebrates tend to have endoskeletons and closed circulatory systems with some level of double circulation, enabling higher metabolic rates.
- Respiration evolves alongside body plans to meet oxygen demands (diffusion in simple forms; gills in aquatic forms; tracheal systems in terrestrial arthropods; lungs with negative pressure ventilation in vertebrates; flow-through lungs in birds).

Quick numerical references from the slides to remember

Megalodon size in the film example: 23 ext{ m} long.
Humpback whale size encounter in the film: 12 ext{ m} long.
Tremors Graboids size (in movie): 9 ext{ m} long and 2 ext{ m} across; weight 10 ext{–}20 ext{ tons}.
Kraken size mentioned in Pirates of the Caribbean 2: at least 20 ext{ m} long (excluding tentacles).
Mothra wingspan in the film: 250 ext{ m}.

Wrap-up reminder

Next week topic: Nutrition and how basic body plans constrain animal lifestyle and body size via the skeletal, respiratory, and circulatory systems (as introduced in this module).