Sponges and Cnidaria

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Sponges and Cnidaria

Of the many phyla of multicellular animals, one is most often

separated by biologists out into its own Kingdom: the sponges'

group,Porifera. There are reasons to think that the protozoa

that led to their multicelled existence were different than those

that led to the rest of the animals, and that alone should put

them in a different Kingdom. Of course, as is always true in

these discussions, none of this matters to the sponges.
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Poriferans are commonly referred to as sponges. An early

branching event in the history of animals separated the sponges

from othermetazoans. As one would expect based on theirphylogenetic position, fossil sponges are among the oldest

known animal fossils, dating from the Late Precambrian. Since

then, sponges have been conspicuous members of many fossil

communities; the number of described fossil genera exceeds

900. The approximately 5,000 living sponge species are

classified in the phylum Porifera, which is composed of three

distinct groups, the Hexactinellida (glass sponges),

the Demospongia, and the Calcarea (calcareous sponges).

Sponges are characterized by the possession of a feeding system

unique among animals. Poriferans don't have mouths; instead,

they have tiny pores in their outer walls through which water is
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drawn. Cells in the sponge walls filter goodies from the water as

the water is pumped through the body and out other larger

openings. The flow of water through the sponge is

unidirectional, driven by the beating of flagella which line thesurface of chambers connected by a series of canals. Sponge

cells perform a variety of bodily functions and appear to be

more independent of each other than are the cells of other

animals.

Sponges are multicellular, but just barely. There are several

species that can withstand being completely pulled apart until

all of the cells are separate, which can re-form a sponge. Unlike

true colonials, the cells can't live indefinitely on their own, but

this re-formation ability is still just a small step above

colonialism. Once formed, sponges cannot move around. They

are sessile, meaning that they generally stay in one place.

About the only ability they have to move comes in their

swimming sperm and in an early, flagellated embryo form.

Sponges do not spread very quickly.


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Sponges are made up of two layers of cells, and ectoderm /

epidermis and an endoderm, with a jellyish but non-cellular

layer between. The ectoderm is a protective outer covering

which is full of pores and channels leading to an inner chamber

of set of channels lined with a type of feeding cell called

achoanocyteor collar cell. A flagellum is surrounded by a ring
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like a comb; the flagellum draws water through the comb,

where tiny particles are caught and carried down to the cell

itself. Food particles are taken in and digested inside the

sponge's cells and the nutrients shared.

Animals that draw water through structures and strain food out

are calledfilter feeders. Sponges are filter feeders, but there

are many other filter feeders in many other phyla. Since they

pass large amounts of water and tend to take in lots of

particles, filter feeders are often used as environmental

monitors - if something in the water goes bad, often the filter

feeders are the first to show the effects.


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Wandering about the jelly layer and positioned between the

collar cells are cells called amebocytes; these do the actual

digesting of food, but they can also crawl to where a cell is

dying or has been lost and change to replace it - they can

become a tough epidermis cell or a collar cell, or a sperm / egg

making cell when necessary. It's just a matter of expressing

different genes to take on the different forms and functions.


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The jellyish layer is reinforced with stiff needles of various

materials. These are called spicules, and the type of material

used in the spicules is the main determinant of which sponge

subgroup a species belongs to: sponges that lack stiff spicules

(and were once used as bath sponges) and are reinforced

mostly with protein fibers; sponges that use glassy and very

pointy silicate spicules, the glass sponges; sponges that use

calcium carbonate (the same material our bones are made out

of).

THE CNIDARIANS - SIMPLE CREATURES THAT STING THINGS TO

DEATH

In many ways, animals from the PhylumCnidariaseem very

similar to sponges: these animals also have an outside

(ectoderm or epidermis) and inside(endoderm or gastrodermis) cellular layer with a jellyish layer

with roaming replacement amebocytes in between, and many

Cnidarians are sessile, with swimming sperm and sometimes

swimming larvae. However, that's about as far as the

resemblance goes.

Cnidarians are almost exclusivelypredators, catching and

eating other animals. The characteristic feature of the groups,

their cnida, are devices through which they capture prey: they
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might deliver a paralyzing sting, a capturing harpoon and cable,

or a tangling net. The poisonous sting, callednematocysts, is

the most common type of cnida. The old name for the phylum

was Coelenterata, a name based upon their internal space, but

there was nothing especially unique about that structure, while

cnidae, the basis for the new name, are not found in any other

group.

Although Cnidarians are often not considered complex enough

to have tissues, they do have strands of cells that act as muscles

and a nervous system, although there are no processors or

"brains." They almost always have a ring oftentacles around

the mouth, which opens to allow food to be pushed into an

inner space called thegastrovascular cavity. The "gastro" part

of the name is from the fact that digestion occurs there

("gastro" = "stomach"), and the "vascular" part is from the

extension of the hollows to all parts of the body for distribution

of nutrients and oxygen, like a vascular (tube distribution)

system. The body around the gastrovascular cavity can be

varied in shape, but it always has a pattern calledradial

symmetry: a circular pattern where the features in any "pizza

slice" will more-or-less duplicate those in another slice the

same size. Some animals, including several types of Cnidarians,

have radial symmetry that follows a specific numeric

pattern; sea anemones, for instance, areoctoradially
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symmetrical, with any one-eighth slice equivalent to any other

one-eighth slice.

Two main body plans can be found in the

Cnidarians: polyps,with the tentacles and mouth on top, and

usually a column-shaped sessile body, andmedusae,with the

tentacles and mouth underneath, and usually a bell-shaped

body that can pulse and swim - a jellyfish. There are some

groups with just one or the other, and some with both, and no

one knows for sure which shape came first - the oldest fossils

include both.

In the Cnidarian groups that have both forms, a type

ofalternation of generationsis followed that allows a sessile

animal to spread great distances. The polyp forms are

the asexual stage, able to make lots and lots of polyps and

spread locally, and to asexually produce the genetically-

identical-but-physically-different medusae. The medusae are

the sexual stage, able to swim away from the parent (and drift

long distances asplankton), find other medusae to mate with,

and drop larvae where they might be able to colonize an

entirely new area, such as near islands newly-risen from the sea

floor.

TYPES OF CNIDARIANS
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Most Cnidarians are marine, or living in ocean / sea

environments; these include many varieties ofcorals, which

commonly secrete a hard, even rocky, material around

themselves, as well as groups that are jellyfish but never polyps

(all of the jellyfish more than about 5 centimeters in diameter

are in this group) and groups that are large polyps but never

medusae (such as the sea anemones). Of the few fresh-water

types of Cnidarians, the hydrais probably the best-known.

Hydras, which can migrate up streams to move into new lakes,

do not have a medusa form, which makes sense in a stream

environment. A few Cnidarians found in large, old lake systems

may still have medusa stages.

Perhaps the oddest Cnidarians are thePortuguese Man-o-

War, a colony of varies polyps that produces a gas-

filled float from which the feeding tentacle hang down. These

drift in the wind, paralyzing and drawing in fish (and occasional

swimmers) down to as much as 30 meters below.

Some Cnidarians produce venom that can kill a human - a

relatively small Australian jellyfish kills more people every year

there than sharks. People also may develop allergies to stings

that can be as severe as bee-sting allergies. When you also

consider that just being stung can panic and disorient someone
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enough to cause them to drown, it's obvious that in many ways

these can be dangerous beasties.

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Sponges and Cnidaria