1. Chapter 7: Marine Invertebrates
Bilateral Symmetry and the Advancements of the Worms

2. Oh, to be a Worm! Adaptive trends exhibited by worm phyla:
Bilateral symmetry
Cephalization –development of a head region
Coelom development
Increasing development of nervous sensory systems.
Many phylum of worms exhibit many new characteristics above the poriferans and cnidarians.

3. Bilateral Symmetry
“Bilateral symmetry refers to a basic animal body plan in which one plane of symmetry exists to create two mirror-image halves.”
Sumich (1999) An Introduction to the Biology of Marine Life
Body divided into roughly identical right and left halves when sliced down the middle.
Considered an adaptation for motility
Planaria
gecko.gc.maricopa.edu/.../platyhelminthes/ platyhel.htm

4. Bilateral Symmetry
Organisms with bilateral symmetry have developed an anterior “head” region and a posterior “tail” region.
In addition they also display a top or back side (dorsal) and a belly or underside (ventral).

5. Worms with Direction
“Animals with a front end [anterior] region generally move in a forward direction.”
Villee, et. Al. (1989) Biology
Thus the tendency would naturally be to concentrate sensory organs in this anterior region to detect changes in the environment.
Leads to more active predation
More sophisticated behaviors
This process is termed “cephalization”
from the Greek for “getting a head”

6. A Bit About Germ Layers
Early in embryonic development, the structures of most animals develop from three tissue layers call germ layers.
Ectoderm – outer layer
Mesoderm – middle layer
Endoderm – inner layer
Ectoderm – outer covering of the body and the nervous system
Mesoderm – gives rise to most of the body structures
Endoderm – lines the digestive tract
Digestive cavity

7. Germ Layers Ectoderm – outer layer Mesoderm – middle layer
outer covering of the body and the nervous system
Mesoderm – middle layer
gives rise to most of the body structures
Endoderm – inner layer
lines the digestive tract

8. A Tube-Within-A-Tube
As organisms become more sophisticated anatomically, the development of a body cavity or coelom [see-luhm] is observed.
The coelom is lined by mesoderm tissue and is essentially an open tube within the organism’s body in which digestive, reproductive and other organs arise.

9. ‘Tubular’ Terminology
Animals can either be
Acoelomate – no body cavity
Pseudocoelomate – a body cavity develops between the body wall (ectoderm) and the internal organs (endoderm). Usually filled with fluid.
Coelomate – the body cavity is completely lined with tissue from the mesoderm.

10. Advantages of a Coelom
It allows for more extensive growth of the organs such as those of the digestive tract.
It permits the formation of an efficient circulatory system with a heart that can drive the blood through the vessels without them being restricted by a compact body.
The fluid in the coelom can transport or move materials faster than by diffusion.
The fluid can also generate a more efficient hydrostatic force against which muscles can act.
The muscles of the digestive tract can become independent of the muscles of the body wall permitting more variation in movement of both sets of muscles.
The coelom provides a space for gonads to develop during breeding season or for young to grow in those animals which give birth to live young.
From Dr. Kent Simmons, Campus Manitoba Web
- It allows for more extensive growth of the organs such as those of the digestive tract.
- It permits the formation of an efficient circulatory system with a heart that can drive the blood through the vessels without them being restricted by a compact body.
- The fluid in the coelom can transport or move materials faster than by diffusion. Animals often dump food or wastes into the coelom and depend on body movement to distribute the materials to the required areas.
- The fluid can also generate a more efficient hydrostatic force against which muscles can act.
- The muscles of the digestive tract can become independent of the muscles of the body wall permitting more variation in movement of both sets of muscles.
- The coelom provides a space for gonads to develop during breeding season or for young to grow in those animals which give birth to live young.
Because of the significance of the coelom, all triploblastic animals passed the Platyhelminthes, have some form of secondary body cavity. The embryonic origin is not always the same. Animal taxonomists have placed a considerable emphasis on the development of the coelom as indicator of relatedness among different groups of animals. In the next section we will deal with a simplified version of this development and the variations that are recognised as having taxonomic importance. Other aspects of development or embryology, are also used in taxonomic schemes and will be considered.
Therefore, having a true coelom means that the muscles that control movement are not the same ones that control digestion or other internal organ systems.

11. PHYLUM: Platyhelminthes Flatworms – A Tiny “Inch” Forward
Exhibit bilateral symmetry and cephalization
Acoelomate
Mouth and anus are still shared
Simplest organisms with well-developed organs
Have a simple brain called a ganglia in the head with two nerve cords that extend the length of the body.
Among the organs are a muscular pharynx, eyespots, and sensory organs in the head – simple brain, complex reproductive organs

12. Flatworms Anatomy of a Flatworm Turbellarians Trematodes Cestodes
Planarians
Marine, free-living
Trematodes
Flukes
Mostly parasitic
Cestodes
Tapeworms
Parasites that live in the intestines of vertebrates (including humans!)

13. Flatworms – Another Look
Anatomical diagram of a planarian – a typical flatworm found in both fresh and marine waters as well as terrestrial habitats
Flatworm Media
Planaria
Swimming Turbellarians
Trematode infection of salamanders
Warning: Colonoscopy showing tapeworm !

16. PHYLUM: Nemertea Proboscis Worms/ Ribbon Worms
Simplest animals to possess definite organ systems.
Almost exclusively marine
Possess a proboscis – a long, hollow, muscular tube which can be everted from the head to capture food or for defense.

17. Proboscis Worms/ Ribbon Worms
Are truly a “tube-within-a-tube.” The digestive tract is a complete tube with mouth at one end and anus at the other.
First example of separate circulatory and digestive systems
Acoelomates
Non-parasitic, mostly benthic
Claim to fame – one species has been observed up to 30 m long (the longest invertebrate!)

18. PHYLUM: Nematoda Roundworms
Most common worms in the world – inhabit almost every species of plant and animal.
Mostly parasitic, some benthic
Have a tough, outer covering called a cuticle which keeps them from drying out.
Sexes separate and dimorphic – separate male and females that look different (male smaller)

19. Roundworms Pseudocoelomates
Have a cavity filled with incompressible fluid which acts as a hydrostatic skeleton.
Cavity is not completely lined by mesoderm.
When muscles in the body wall contract they flex and squeeze against this fluid causing the shape of the worm to deform and therefore move.
Excellent technique for sediment burrowing.
Good slide show of various roundworm images
Marine roundworm
Roundworm in
cat gut

20. PHYLUM: Annelida Segmented Worms
20,000 species including marine and terrestrial species (e.g. earthworms)
Defining characteristics
Body divided into segmented units called metameres.
Chaetae (or setae) – hair-like structures on each segment

21. Other Innovations of Annelids
Digestive tract (or gut) extends through all segments.
Coelomates
Acts as a hydrostatic skeleton
Organism can move each segment individually. This permits localized and more efficient movement.
Have a closed circulatory system
In aquatic species, respiratory exchange is through gills

22. Annelid Classes Polychaeta Oligochaeta Hirudinea
All marine, may be free-swimming or live in benthic aggregations
Include bloodworms, sandworms, lugworms, bristle worms, fan worms, feather duster worms, beard worms, etc.
Oligochaeta
Aquatic or terrestrial, live in mud or sand bottoms’
Include earthworms
Hirudinea
Mostly freshwater, but some marine species
Leeches

23. Polychaete Biology Anatomy: Life History: Feeding:
Chaetae emerge from flat parapodia which are stiff extensions on each body segment
Life History:
Have a planktonic larval stage called a trochophore
As adults, some crawl on bottom, others burrow, others build tubes and live in aggregations, while still others remain planktonic
Feeding:
Some are carnivorous, some are suspension feeders, and others are deposit feeders.
Crawling worms have well developed parapodia, a proboscis, and jaws.
Suspension feeding worms often have tentacles, cilia, or mucus to capture prey

24. Serpula vermicularis – reef building tube worm

25. Common lug worm (Arenicola marina) Plymouth, Devon, England

26. Lug worm casts on the coast of North Ireland

27. King Ragworm (Nereis virens)

28. Tubeworm (Spirorbis tridentatus) Batten Bay, Mount Batten, Plymouth, Devon.)

29. Myrianida pachycera, a polychaete (worm) (60x)

30. Christmas tree worms on coral head

31. Trochophore larvae of a bristle worm
Note the bristles anchored in the body for swimming and the reddish eye spots.

32. Polychaete sandworms - Notice the tubes sticking up from the mud.
Some sandy beaches can contain up to 32,000 polychaete worms/m2 that consume 3 tons of sand/ year.

33. Feather duster worms, Bimini, Bahamas

34. Polychaete epitokes swarming . Glover’s Reef, Belize

35. Pogonophora beard worms
Deep water species – live near hydrothermal vents
No mouth or gut
Tuft of tentacles absorbs dissolved nutrients from the water
Symbiotic bacteria inside the worm use these nutrients to make food.
Formerly classified in their own phylum

36. Oligochaeta Found in mud/sand bottoms Usually deposit feeders
Lack parapodia
Includes the common earthworm

37. Hirudinea leeches Usually parasitic and blood-sucking
Inject a chemical into prey that is both an anticoagulant and an anesthetic.
Have a sucker on anterior and posterior.
Lack parapodia

38. Sipuncula peanut worms
Strictly marine
Unsegmented
Burrow in shallow water soft bottom sediments
Possess a long anterior portion that can be retracted into the body.
Deposit feeder
1-35 cm long
Approximately 320 species

39. Echiura innkeepers/ spoon worms
Strictly marine
Unsegmented, though now classified with annelids
Have a non-retractable, spoon-like proboscis for gathering organic material.
One species creates a U-shaped burrow that is often shared with other organisms.
Deposit feeder
Approximately 135 species
proboscis

40. Unifying Characteristics of Worms
Ubiquitous in marine environment (benthic, parasitic, free swimming)
Usually small
Responsible for mixing marine sediments.
Recycle bacteria and detritus into the food chain.
Have highly developed feeding appendages and digestive systems.
Important food for higher invertebrates and some fish.
May have important health effects on marine vertebrates

41. Image Citations http://www.aphotofauna.com/page27.html
Brown, Hugh. “Serpulid polychaete worm” Digital Image. Serpulid reefs. The Scottish Association for Marine Science (SAMS). 5 January < Fiege, Dieter. “Glyceridae” Digital Image. Senchenbergische Naturforschende Gesellschaft January < “Leech.” Digital Image. Annelids Live Invertebrates – Niles Biological, Inc Niles Biological, Inc. 5 Jaunary 2009 < Rouse, Greg. “Chaetae of an Annelid” Digital Image. Annelida Tree of Life Web Project. 5 January 2009 < Rouse, Greg. “Myrianida pachycera, a polychaete.” Digital Image. Nikon Small World – Gallery Nikon Small World – Photomicrography Competition. 5 January < Siddal, Mark. “Medicinal leech” Digital Image. Leech on Me Science Friday Newsbriefs. 5 January < “Social feather duster worm close-up” Digital Image. ReefNews January “Swarming polychaetes” Digital Image. Rpolychaete epitokes Ryan Photographic. 5 January < “Trocophore larvae” Digital Image. Bristleworms and their larva Mic-UK: Bristle worms. 5 January < Veitch, Nick. “Lug worm casts” Digital Image. Wikimedia Commons January <