An Online Introduction to the Biology of Animals and Plants


Key Concepts


Section 3

Chapter  2

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.

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 cant 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.

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 a choanocyte or collar cell.  A flagellum is surrounded by a ring 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 sponges cells and the nutrients shared.

that draw water through structures and strain food out are called filter 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.

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.  Its just a matter of expressing different genes to take on the different forms and functions.

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).




In many ways, animals from the Phylum Cnidaria seem 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 larvaeHowever, thats about as far as the resemblance goes.

are almost exclusively predators, catching and eating other animals.  The characteristic feature of the groups, their cnida, are devices through which they capture prey:  they might deliver a paralyzing sting, a capturing harpoon and cable, or a tangling net.  The poisonous sting, called nematocysts, 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 of tentacles around the mouth, which opens to allow food to be pushed into an inner space called the gastrovascular 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 called radial 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, are octoradially 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, and medusae, 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 of alternation of generations is 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 as plankton), 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.




Most Cnidarians are marine, or living in ocean / sea environments;  these include many varieties of corals, 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 hydra is 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.

the oddest Cnidarians are the Portuguese Man-o-War, a colony of varies polyps that produces a gas-filled float from which the feeding tentacles 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 enough to cause them to drown, its obvious that in many ways these can be dangerous beasties.


Informational Links




Lots more on Sponges.

A database of Caribbean sponges.


Click on term to go to it in the text.
Terms are in the order they appear.


Sponge Features

Ectoderm / Epidermis
Choanocyte / Collar Cell 
Filter Feeding 
Cnidaria-Sponge Similarities 
Cnidaria Characteristics 
Gastrovascular Cavity 
Radial Symmetry 
Alternation of Generations 
Portuguese Man-o-War 
Human Impacts


Go On to the Next Section - Flatworms



Online Introduction to the Biology of Animals and Plants.

Copyright 2001-2019, Michael McDarby.   e-mail Contact.

Reproduction and/or dissemination without permission is prohibited.


Hit Counter