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Invertebrate Skeletons
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Covers some variations in skeletons, and how the bones form. It also includes different types of bone in invertebrates
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Comparative Anatomy and Physiology of AnimalsLecture 19 Invertebrate Skeletons 05/01/21
Animal body types
- Acoelomate: no fluid-filled cavity. Found in animals such
as flatworms. It is unsure whether it is primitive, or
secondarily lost.
- Pseudocoelomate: contain pseudocoelom (a fluid-filled
space enclosed by muscle on the outside only). These
animals evolved from coelomates.
- Coelomate: contains coelom. Here, the body cavity is
within the mesoderm, and is lined with the peritoneum.
Skeletons
- A skeleton is any structure that maintains body shape,
supports and protects a body, and transmits contractile
forces.
- There are 3 different types of skeletons: hydrostatic,
exoskeletons, and endoskeletons.
Hydrostatic skeletons
- Fluid skeleton.
- Supported by fluid pressure.
- Hydrostats have constant volumes, which transmit
muscle contractile forces.
- Examples include polyps and vermiform (worm-like)
animals.
- Body walls reinforced with a mesh of inelastic fibres.
- Orthogonal pattern: does not allow for changes in length, bends
until failure from kinking, allows for torsion.
- Cross-helical pattern: allows for changes in length, bends in a
curve and resists torsion.
- Muscle structure and movement: contains both longitudinal and
circular muscles. Muscles can only contract, not push. Localised
muscle contraction displaces fluid to another part of the body,
where muscles are relaxed.
- Movement using a hydrostatic skeleton: circular muscle at
posterior end contracts. This forces fluid forwards and extends
the front of the animal. Longitudinal muscles then contract to
pull the posterior end forwards.
- Segmentation: in annelids, the coelom is divided into segments by muscular septa. This
prevents movement of fluid from one segment to another. This allows the individual
segments to operate independently, allowing for both more complicated and variable
patterns of movement, and increased protection from injury.
Molluscan shells
- Molluscs have hydrostatic skeletons but also have a mantle that secretes a protective
calcareous shell.
- The shell consists of three layers: periostracum, prismatic, and nacreous layers.
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