1. What is a Metazoan?
In other words what are the inherent
Characteristics of the animal body plan that are different from that of a colonial heterotrophic protists?
What are some characteristics of sponges that help us place them directly in the metazoan

2. Phylum Placozoa -- 2-3 mm, 25 um- thick, resembling a large ameba
-- Lacks anterior posterior polarity
-- Asexual reproduction is prevalent
--The most primitive animal?
Trichoplax adhaerens

3. Phylum Placozoa Dorsal cilium epithelium-like layer Fiber synctium
thick glandular layer
Flagellated cells
Intercellular
junctions
- Feed ventrally by absorption of digested material
- Lack organs but tissue-like outer walls
- A bit more than 2000 cells
- Only 4 different cell types
(20 in sponges; > 220 in mammals)
- Smallest genome of all animals

4. Three competing Scenarios
Functional Morph.
Epitheliozoa Eumetazoa
Earliest view of them as the basal metazoan
Special cellular junctions consisting of two opposing dense plaques (desmosomes) not found in most sponges
16S rRNA data…maybe secondarily simplified from more complex ancestors?

5. Mitochondrial genome of Trichoplax adhaerens supports Placozoa as the basal lower metazoan phylum
Dellaporta, Stephen L. et al. (2006) Proc. Natl. Acad. Sci. USA 103,
Largest known mt genome 46 kbp, 2x that of most metazoa
with introns and other intrageneic spacers and large protein coding regions that are usually lacking in other animals
Blue: known mt proteins
Gray: ribosomal genes
Green: unknown open reading frames
Red lines: introns
ORF is an open reading frame, a section that encodes for proteins.
The proteins of the ORF shown for Placozoa in green are unknown.

6. (a sponge)
Conclusions?

7. Phylum Cnidaria
More than 9, 000 species, including jellyfish, corals, anemones, hydroids and much much more.

8. Themes General characteristics of Cnidaria
Why are they successful?
Cnidaria Diversity and Taxonomy
Classes, SubClasses of Anthozoans
Similarity of Polyp and Medusa
The Cnidaria Body Plan
Class Scyphozoa, the jellyfish
Complexity, Sensory & Nervous System

9. What Characteristics Account for the Great Success of Cnidarians?
1. Two distinct adult forms
2. Diphasic life cycle in some species
3. Ability to proliferate by cloning
4. And to form polymorphic colonies
5. Formidable prey capture device
6. Low Energy Demands

10. What Characteristics Account for the Great Success of Cnidarians?
1. Two Very Different Adult Forms
Polyp
Medusa

11. What Characteristics Account for the Great Success of Cnidarians?
2. Dimorphic Life Cycle in Many Species
Life cycle
Of the
Hydroid
Obelia

12. What Characteristics Account for the Great Success of Cnidarians?
3. Ability to proliferate by cloning

13. What Characteristics Account for the Great Success of Cnidarians?
4. And to form polymorphic colonies
Hydractinea

14. What Characteristics Account for the Great Success of Cnidarians?
4. And to form polymorphic colonies
Physalia
Portuguese
Man-O-War

15. What Characteristics Account for the Great Success of Cnidarians?
5. Formidable prey-capture device
Nematocyst
Prey tissue
Empty Cnidocyte

16. What Characteristics Account for the Great Success of Cnidarians?
5. Formidable prey-capture device

17. What Characteristics Account for the Great Success of Cnidarians?
6. Low Energy Demands

18. Class Cubozoa contains
Diversity of Cnidarians
3 Main Taxonomic classes
Class Scyphozoa
Class Anthozoa
Class Hydrozoa
Class Cubozoa contains
a few jellyfish spp

20. How is it that organisms as different
as jellyfish, hydroids and anemones
can be classified in the same phylum?
Giant anemone

21. Similarities of Polyp and Medusa

22. Epitheleo-muscular cells
Nutritive
muscular
cells
Gland
Epitheleo-muscular cells
cnidocytes
mesoglea

23. Nerve Nets and Muscular System
Mouth

24. Epitheliomuscular Cells and gastromuscular cells
Lacking mesoderm, but muscles derived
from epithelial endodermal and ectodermal cells

25. Characteristics of the Class Scyphozoa
(Adaptations to a drifting, carnivorous lifestyle)
About
200
species

26. Hydromedusa for comparison Small a few cm Velum to aid in locomotion
Radial canals unbranched
Simple epidermal and endodermal nerve net
Nematocysts usually small without toxins

27. Characteristics of the Scyphozoa
mesoglea
gastric pouch
stomach
gonad
radial canal
gastrodermis
Radial
canal
mouth

28. Characteristics of the Scyphozoa
(Adaptations to a drifting, carnivorous lifestyle)
Stomach
Stomach
G-V cavity more
complex, adapted to subdue and
digest larger and active prey
G-V Canals
Gastric Pouches
MOUTH
MANUBRIUM

29. Characteristics of the Scyphozoa
(Adaptations to a drifting, carnivorous lifestyle)
G-V cavity more
complex, adapted to subdue and
digest larger and active prey

30. Characteristics of the Scyphozoa
mesoglea
gastric pouch
stomach
gonad
radial canal
gastrodermis
Radial
canal
mouth

31. Coronal Muscles and swimming
According to literature, in large scyphozoan medusae the epidermal muscles predominate while the gastrodermal muscles are relatively reduced. This makes sense in that the gastrodermal muscles are usually the circular muscles, and their action is perhaps transferred to the coronal muscles. The epidermal muscles are the longitudinal bands.
The coronal muscles are described as “circular sheets of “epidermal muscle”
In some cases during development cells become separated from the epithelial layes and move into the mesoglea where they develop into “true” muscles…perhaps this is where the coronal muscles come from?
How is swimming pace, direction coordinated?

32. How are the coronal muscles connected to the nervous system that includes:
Sensory neurons, motor neurons
Nerve net
- Inner and outer nerve rings
- Marginal ganglia associated with rhopalia that constitute a pacemaker

33. (Adaptations to a drifting, carnivorous lifestyle)
To ganglia
Characteristics
of the Scyphozoa
(Adaptations to a drifting, carnivorous lifestyle)
The nervous system is more complex; unique sensory structures
(than nerve net shared by all Cnidaria)

34. Rhopalium
(Digestive)

35. Rhopalia are Intergrated with Nerve Rings in Box Jellies (Cubomedusa)
Garm et al., 2006
Cell Tissue Research #325
EN: epidermal nerve
GN: gastrodermal nerve
RC: Ring Canal
EN joins the nerve ring
NCB: nerve cluster
RN: ring nerve

36. 24 eyes but no brain? Epidermal nerve of rhopalia Inner and outer
nerve rings
Marginal
ganglia
Nerve net
Coronal muscles
Epiheliomuscular system

37. When did a Centralized Nervous System Evolve?
“Conventional” view
Marlow et al., 2009, Developmental Neurobiology

38. Cnidarian nerve net e.g. Hydra

39. “Sub-functionalized” N.S.
Sea Anemone Nematostella

40. Nerve rings are Core of CNS in medusae
-- Concentrations of hundreds of axons in parallel form coupled nerve rings. Having an annular form rather than a large ganglion does not make it less centralized.
-- The annular configuration is a function of radial symmetry
Mackie, Neurosignals #13