Jellyfish and Sponges: Simple Organisms
The simplest groups of animals, sponges and cnidarians, live in seas and fresh waters. Sponges are sessile and live in colonies at the bottom of waters. The spicules that support their soft tissues are composed of calcium carbonate, silica or keratin-like proteins. Cnidarians are free-floating
or sessile predators named after their cnidocytes.
Locomotion
The water is squeezed
from the bell-shaped body
of the jellyfish, propelling it
in the opposite direction
of the water flow, much like the propulsion of a rocket.
Cnidocytes
As soon as the prey
touches the ciliary
receptors of the cnidocyte,
it becomes active: the operculum opens, the cnidocil penetrates the
body surface of the prey, and a thread is fired, which injects venom
into the wound.
This process takes
place in less than
a microsecond.
Oral arms
They contain lots of cnidocytes. They play an important role in capturing the prey and
directing it into the gastric cavity.
at rest
cnidocil
thread
toxin
epidermis
central cavity
Jellyfish
Jellyfish are free-swimming predatory cnidarians. They paralyse their prey using their cnidocytes. Jellyfish use their tentacles (or oral arms) to direct the prey into the gastric cavity via the mouth. Waste is also excreted via the mouth.
Sponges
Sponges are the simplest multicellular organisms; they lack true tissues. Water flows through
the pores in their bodies, into the central cavity. Sponges filter food from the water flowing through them. Extra water is expelled through the osculum.
Did you know?
- If the cells of a sponge are separated by shaking, the cells will come together again to form a new individual.
- Irukandji jellyfish are the most venomous jellyfish in the world; their venom is estimated to cause the death of around a hundred people every year.
- The cleaned exoskeleton of certain sponges is suitable for human use. Sponges are collected in spring by sponge divers who use knives or sponging rakes. The sponges are rinsed and pressed several times then washed with hot washing soda solution and with diluted hydrochloric acid. After that they are bleached. Before they are sold, they are cut to the desired shape and separated by colour.
Invertebrates
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Invertebrates
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