On Being a Worm

1. On Being a Worm

2. Worms? • Any soft-bodied, legless animal whose length exceeds its width is often described as a worm

3. Why are there so many different kinds of worms? • Easy shape to evolve • Mechanical facts about molecules worm shape • Cells produce extracellular cpds. With charged molecular backbones (ex. proteins) • Like charges repel causing linear extension • Linkage between these large molecules provides orientation in a structure that has different properties in different directions (anisotropic) • If blob of soft tissue has these fibers, any event such growth or motion or external pressure will turn the blob into a cylinder. Needs no other genetic instruction to make worm shape

4. Why is the worm shape favored by natural selection? • Soft tissues are resistant to damage, and any damage is readily repaired • Large range of possible sizes • Less than a mm to over 30 meters in the sea • Movement in one direction will be favored by worm-like shape • Anterior end is established • At least a simple brain and sense organs • Distinction between dorsal and ventral confers bilateral symmetry Bilateria evolve

5. Why is the worm shape favored by natural selection? • Worms are triploblastic • Have mesoderm between ectoderm and endoderm • Important difference from diplobastic cnidarians • Separates muscle fibers from cells of ectoderm and endoderm • Allows muscle fibers to run in all directions • Most worms move my muscle contraction • Planaria and a few other taxa use cilia

6. How can muscles move a worm? • Hydrostatic skeletons • Water is not compressible: closed bag of fluid can change its shape but not its volume • Soft-bodied animals lacking hard skeletons use this fact to re-extend their muscles mostly using the filled-filled body cavity

7. How can muscles move a worm? • Hydrostatic skeletons • Uses: • Burrowing – in many worms, circular contraction extends the anterior end which takes hold; then longitudinal contraction pulls up the posterior part • Ex. Earthworm (annelid)

9. How can muscles move a worm? • Hydrostatic skeletons • Uses: • Wave motion – Waves of contraction are propagated alternately along the sides of the worm producing and S shape; this puts backward pressure on the substrate so the worm moves forward • Waves of longitudinal muscle contraction pass posteriorly – Longitudinal muscles on each side of body contract out of phase producing a rapid wriggling motion • Ex. Nematodes, Polychaete annelids

10. P = power stroke of setae R = recovery stroke of setae

11. How can muscles move a worm? • Hydrostatic skeletons • Uses: • Jet propulsion – Normal swimming: mantle cavity is expanded by elastic recoil of stretched collagen fibres extending through the mantle wall; these are then contracted rhythmically forcing water out through the hyponome. Fast escape swimming: circular muscles are used to hyperinflate the mantle cavity, increasing its volume and thus the amount of water that can be expelled. • Ex. Octopus, squid, cuttlefish

12. How can muscles move a worm? • Muscular Waves: • Uses • Ex. Marine flatworms • Ex. Snail foot • Ex. Starfish tube feet • Muscles push fluid

13. Body Cavities • Some worms have no cavity • Acoelomate: Flatworms = Platyhelminthes • Some worms have a cavity • Coeloms: ex. Annelids, Chordates, and Echinoderms • Pseudocoels: ex. Rotifers, Nematodes • Hemocoels: ex. Molluscs

14. Lattices • Hydrostatic skeletons need the support of connective tissue lattices • Threads of collagen are wound helically around the soft body • Animal can therefore contract evenly without bulging or kinking as the lattice angle changes

16. Disadvantages to Hydrostatic Skeleton • Disadvatages • Requires water! • Animal must either be aquatic or it must live in moist habitats • Weight of fluid required offsets the advantage of not having a heavy hard skeleton • lattice threads are very light and thin • Requires lots of muscle • To move entire body requires lots of muscle • Locomotion is uneconomical in terms of energy • Whole body must be accelerated and decelerated

17. Disadvantages to Hydrostatic Skeleton • Disadvatages • System is hard for nerves to control and impossible to control precisely • Ex. Beetle knows where each leg is and the degree of bend at each joint. Worm has no such information. • Animal is vulnerability to predation

18. Disadvantages to Hydrostatic Skeleton • Disadvatages • System is hard for nerves to control and impossible to control precisely • Ex. Beetle knows where each leg is and the degree of bend at each joint. Worm has no such information. • Vulnerability to predation

19. What form phyla are known? • Major phyla • Platyhelminthes • Nemertea • Nematoda • Annelida • Minor phyla • Rotifera • Gastrotricha • Kinorhyncha • Numerous others