1) Diploblastic with ectoderm and endoderm separated by acellular mesoglea

2) Radially symmetrical, no cephalization

3) Unique stinging/adhesive structures called nematocysts that are within cnidocytes

4) Gastrovascular cavity (coelenteron) with one opening

5) No excretory, circulatory or respiratory systems. No centralized nervous system

6)Alternation of generation between asexual polyp and sexual medusa forms

7 Planula larvae

Review general characteristics of Cnidaria

1) Cnidaria seen in lab last week, unlike Poriferans have true germ layers represented by only ectoderm and endoderm. Therefore they are considered diploblastic. After recent morphological studies of Ctenophorans, they may represent the only phyla that is diploblastic-but this hasn’t been resolved yet.

2)Cnidaria are radially symmetrical.-for this reason they are often referred to as the "Radiata".

As I mentioned before with respect to symmetry patterns, the reality of the animal is that most are not truly radially symmetrical,

For example:

SHOW OVERHEAD (218 BRUSCA)

The radial symmetry of many Cnidaria been expanded to include quadriradial symmetry (A) and Biradial symmetry of sea anemones and coral.

3)Cnidarians are unique in having stinging/adhesive structures called nematocysts. These are specialized organelles within cells called cnidocytes or cnidoblasts. These are a synapomorphy unifying the Cnidaria as a a monophyletic group. They are also the structures for which the Phylum gets it’s name. I’ll talk a bit about how these work soon but these are used in food gathering and defense.

4)The animals are characterized by a gastrovascular cavity or coelenteron that has only a single opening-so it pulls double duty as a mouth and anus. Hopefully you witnessed the live sea anemones in class and how they can almost turn inside out. this is a way to egest leftover food particles. The gastrovascular cavity is formed during the gastrulation process-but never really gets much beyond that.

5)Cnidaria have no excretory, circulatory, or respiratory systems. Often times they may extrude their gastrovascular cavity to "vent the stale water" within and increase contact with more oxygenated water. As in the Poriferans, the mesoglea is a gelationous base (the jelly in jellyfish) to which the two germ layers attach and can bypass complex organs for respiration and excretion because of the usually acellular nature of the mesoglea. They can also attain a much larger size than they otherwise could by simple simple diffusion.

Cnidarians have a true nervous system to the extent that it involves neurons and electrical conduction-but the nervous system is not centralized. Instead it is composed of a simple nerve net without cephalization or concentrations of nervous ganglia. Touching the Cnidarian at one point will set of electrical impulses in all directions away from the point of contact.

6) Cnidarians exhibit an alternation of generation between an asexual polyp stage and a sexual medusa form. This is a synapomorphic feature shared by all cnidarians but some have secondarily lost the polyp or medusa form.

7) All Cnidarians have what is called a planula larval form.

SHOW OVERHEAD (Figure 4, page 219)

Let’s start by reviewing Cnidarian body plans from a cellular level.

This represents a longitudinal section through a hydrozoan polyp. The hypostomial region or mouth region is up here. the gastric region within the gastrovascular cavity is here, the stalk, and basal or pedal disc.

Cnidarian cells still do their own nutrient uptake and gas exchange-much the way that poriferan cells take in food.

There are epitheliomuscular cells (called myoepithelial cells in Ruppert & Barnes) These are the cells that, as the name implies, serves as both a covering and for muscular contraction. Since Cnidarians don’t have mesodermal tissue, the muscular cells are born from ectoderm tissue.

There are gastrodermal cells are secretory. They secrete both digestive enzymes and the substance that makes up the mesoglea.

Like choanocytes, they are involved with absorbing food, circulation, and food transfer.

Nervous cells are loosely organized into a net within the epithelium and appear to be non-directional in their responses to stimuli.

There are interstitial cells that lie embedded between epitheliomuscular cells. They are undifferentiated cells and are important in regeneration. They serve different functions depending on their location.

In basal disc they secrete mucous and adhesives and contribute to the lengthening the cnidarian-especially in polyps.

In tentacles they create cnidoblast cells and are concentrated at the base and differentiate outward.

I’d like to discuss cnidocytes in more detail since they represent a unique feature among cnidarians.

Nematocysts are the specialized organelle that is actually "fired" within a cnidocyte and it is created by the golgi bodies within interstitial cells.

As cells divide, intercellular bridges remain and nematocyst development is synchronized because the cells are not completely divided, they occur in groups of 16.

Farther out on tentacles, more developed nematocysts occur and are replaced by new ones as they fire.

SHOW OVERHEAD (page 107 Ruppert & Barnes)

One question is how much control over nematocyst firing is there? Is it under Cnidarian control, or

The presence of nerve fibers among cnidoblast cells/interstitial cells argues for nervous system control.

However, the proximal cause is purely physical-or so it appears that environmental stimuli rather than nerve firing sets it off.

Anyone who has been stung by a dead jellyfish on the beach will attest to this. It can’t be under nervous control if you can be stung by dead ones!

Discharge of nematocysts have been found in response to heat and tactile stimulation.

(Godknecht & Tardent, 1988)

There have been proposed three hypotheses about how these stinging structures fire:

1) Contraction (a constrictable net around the cnidocyte or nematocyst)

2) Osmotic pressure (change in membrane permeability to water)

3) Tension (nematocyst is like a coiled spring with a trigger on it).

Ultrastructure studies show that during development, the thread is formed first, then drawn into the coiled position.

Barbs are formed in reverse position

The tube is hollow, and at the distal end contains vesicles of toxin or mucous or whatever.

No evidence for any kind of constrictive musculature or processes surrounding nematocysts to serve as "squeeze" mechanism

So, H1 constriction hypothesis is falsified

However, there is evidence for both other hypotheses.

SHOW OVERHEAD OF NEMATOCYST Before & After firing (Page 236 Brusca)

Nematocysts are among the most complex intracellular structures in the animal kingdom.

The basic morphology is this although it is highly variable:

There is an operculum or capsid on the top that serves as a hinged lid.

The operculum is thrown open during discharge.

The outside the the nematocyst has a Cnidocil.

The cnidocil serves as a mechanoreceptor or a trigger for release. It is a cilium-like fused bristle that will elicit discharge when stimulated.

There is some variation on this theme. Anthozoan cnidarians lack a cnidocil trigger and operculum and instead a tripartite flap with a cilium at the end that acts as a mechanoreceptor.

Electron microscopy studies have examined the meantocysts before and after discarge as well as during development.

Before firing a tightly coiled filament is clearly visible.

During firing, the cap (capsid) opens and the structure within turns inside out.

After discharge there was no change in the size of the cnidocyte, but 1.5-two-fold increase in the bulb volume and 3 fold increase in the tube.

A .17 increase in diameter but a 1.82 change in length of the filament was noted.

This supports an osmotic pressure hypothesis

The walls and thread of nematocysts are made of proteins complexed with polysaccharides (mucous polysaccharide)

These substances bind with water, when the capsule lid opens, they swell and create osmotic pressure to evaginate the thread.

During development, the tube and membranes are much smaller than they are after hydration

Nematocyst formation is really by a dehydration process.

The third hypothesis, the coiled spring hypothesis has been examined as well.

The tube is coiled in a helix-like arrangement. High speed film shows that uncoiling creates a propulsive force by rotation and extension.

Since these nematocysts are unique to the Cnidarians, there has been some debate as to the evolutionary origin of these structures.

Explosive organelles are not unique even if the precise morphology of the nematocyst is:

Of the different types of explosive organelles, all appear to have been derived from mucous polysaccaride explosive hydration-although arguably few phyla have incorporated nor exhibited the diversity of form in these organelles to the degree that Cnidarians have in their functional morphology or feeding habits.

Various ciliated protists have trichocysts

Ctenophorans have colloblasts which are sticky-entanglers.

and Platyhelminthes have rabdites which are exploding mucous structures

It has been suggested that nematocysts are specialized structures for capturing planktonic crustacea-which are evolutionarily "new" compared to Cnidarians.

Did nematocysts evolve later? or did they come from more specialized organelles.

I mentioned that nematocysts are diverse among the Cnidaria...and they are...

SHOW OVERHEAD OF NEMATOCYST TYPES.

(page 132 Coelenterate biology)

There is a hideous amount of terminology associated even with the different types of nematocysts. This shows a representative sample with the representative horrific nomenclature. Make sure you know all these different types-just kidding.......actually there are 20 different kinds in all. Several different kinds within a single individual often. Hydra, for instance has 4 different kinds of nematocysts.

Actually, I’m more concerned that you realize that there are a few general categories of nematocysts.

SHOW HANDMADE OVERHEAD

1)-Toxin containing (Stenoteles)

These are penetrant nematocysts with barbed spines. The toxin is delivered through a pore in the thread.

neurotoxins-paralysis, death, leg autonomy (crabs)

Hemotoxin-lysis blood cells.

Myotoxin-causes severe muscle cramps, etc.

2) Entanglers (Desoneme)

threadlike fibers

tiny hooks

may be used in locomotion (somersaulting hydra)

These nematocysts respond to mechanical stimulation.

3) Sticky (Atrichous isorhiza)

create mucus or adhesive substances for anchoring or catching food.

these are fired by long duration mechanical stimulation.

4) Defensive mucous/chemical discharge (Holotrichous isorhiza)

responds to mechanical stimuli.

These types of nematocysts are fired under different conditions and circumstances.

They serve different functions

The fact that they have different functions implies some control of firing on the part of the cnidarian.

Researchers have found cholinesterase and other enzymes found near nematocyste.

Some nematocysts require chemical stimulus to fire.

The various types of nematocysts of hydra do not fire uniformly.

The stinging nematocysts with venom (Stenoteles) always fire. Kill kill kill, but if you take prey away after killing after a few times, Hydra will fire the desoneme or tangling threads. It is unclear how the hydra "knows" to do this.

Nematocysts have been observed to have other functions. The Portugese Man-o-war’s batteries of nematocysts resemble copepods and fish larvae. There is good morphological and behavioral evidence that these weapons serve as aggressive mimics, attracting prey that normally feed on zooplankton .

CLASSES OF CNIDARIA

Medusoid stage dominant

No velum

Cellular mesoglea

Gastrodermal gonads

Marine only

Cube shaped bell,

Four rhopalia

Velum

Marine only

Polyp and medusoid stages

Medusa with velum

Mesoglea acellular

Gonads are epidermal

Marine or freshwater

No medusoid stage

Cellular mesoglea

Gonads endodermal

Septa in gastrovascular cavity

Solitary or colonial