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Name: Francesca
Status: educator
Grade: K-3
Location: Outside U.S.
Country: USA
Date: Spring 2012

Dear Scientist, In our 2nd grade class, we read a story called Jellies. We learned that jellyfish do not have a heart or brain. If they don't, why are they considered an animal and not a plant?

How did scientists come to the conclusion that they are NOT plants? Thank you for any information you can give us!

Sincerely, Mrs. Holm's Second Graders

Jellyfish move, and they don't make their own food. Plants don't move, and they do (mostly) make their own food.

Tim Mooney


Here we are talking about the classification of organisms into taxonomic rank. Since Biology is not my field of expertise, I got the information for your answer from this URL to answer your quest

“The classification of living things into animals and plants is an ancient one. Aristotle (384–322 BC) classified animal species in his work The History of Animals, and his pupil Theophrastus (c. 371–c. 287 BC) wrote a parallel work on plants (Historia Plantarum (The History of Plants)).[1]


“From around the mid-1970s onwards, there was an increasing emphasis on molecular level comparisons of genes (initially ribosomal RNA genes) as the primary factor in classification; genetic similarity was stressed over outward appearances and behavior.”

“Since 2004, there are six recognized kingdoms: Bacteria, Protozoa, Chromista, Plantae, Fungae, Animalia”

So, as in everything else today, biological classification has become far more complicated than just classifying things as plants and animals. The classification of organisms as a plant or animal depends on the genetics.

The Animal Kingdom is taxonomically explained in this article:

There are a whole list of characteristics which result in classification of an organism as an animal which includes


Reproduction and Development

Food and Energy Sourcing

Why jellyfish are classified to be animals is described in this section:

“Among the other phyla, the Ctenophora and the Cnidaria, which includes sea anemones, corals, and jellyfish, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus.[58] Both have distinct tissues, but they are not organized into organs.[59] There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, these animals are sometimes called diploblastic.[60] The tiny placozoans are similar, but they do not have a permanent digestive chamber.”

Sincere regards, Mike Stewart

Hi Francesca,

First: Thank You for teaching second grade! Your patience must be endless. My second grade teacher, Mrs. Clower, led me to realize wonderful science. There are many teachers like you I am truly indebted to. Thanks, Peter

Now for your question: The confusion lay in the belief that differences at the organ level classify an animal or plant. In fact, flowers, buds, rhizomes and more are functional organs of plants. The difference for classification of plant-animal is much more basic – the differences are at the basic biochemistry and the cell levels.

Plants have tend to be rooted or float as a mass, have cell walls and make their own food through photosynthesis – conversion of carbon dioxide and sunlight to energy, biochemicals and oxygen.

Animals move freely usually with locomotion(although rudimentary), they must hunt for food, and must convert biochemicals and oxygen to form energy and carbon dioxide. Animals have no rigid cell wall.

Based on this classification, jellies are animals: They create their own rhythmical motion, they hunt for food with their tentacles and nematocysts, they digest the food to form energy.

Hoping this helps! Peter E. Hughes, Ph.D. Milford, NH

Hello Francesca,

My on line dictionary says it well.

animal |ˈanəməl|nouna living organism that feeds on organic matter, typically having specialized sense organs and nervous system and able to respond rapidly to stimuli Animals are generally distinguished from plants by being unable to synthesize organic molecules from inorganic ones, so that they have to feed on plants or on other animals. They are typically able to move about, although this ability is sometimes restricted to a particular stage in the life cycle. The great majority of animals are invertebrates, of which there are some thirty phyla; the vertebrates constitute but a single subphylum.

plant |plant| noun1 a living organism of the kind exemplified by trees, shrubs, herbs, grasses, ferns, and mosses, typically growing in a permanent site, absorbing water and inorganic substances through its roots, and synthesizing nutrients in its leaves by photosynthesis using the green pigment chlorophyll.• a small organism of this kind, as distinct from a shrub or tree : garden plants. Plants differ from animals in lacking specialized sense organs, having no capacity for voluntary movement, having cell walls, and growing to suit their surroundings rather than having a fixed body plan.

So you see the jellyfish can fit into the animal definition because they can move voluntarily, they do not have cell walls and they eat organic matter. I hope this is a simple enough explanation for the second graders. If they have more questions, please feel free to write back.

Happy Science! Martha Croll

The simple answer is animals have to ingest other organisms or products thereof for food whereas plant produce their own food through photosynthesis. There are other characteristics that differentiate Kingdom Plantae for Animalia such as motility but the food issue is the simplest differentiation.

Jellyfish eat other organism and do not utilize photosynthesis for energy. The are many animals without those organs...a sponge for example. While jellyfish don't have brains, they do have nerves which help them detect and respond to changes in their environment.

Dr. Horne

The definition of an animal is: a multicellular, heterotrophic eukaryote with no cell walls. "Jellies" fit that characteristic. Many people confuse animals with mammals, which are a type of animal.


The single defining difference between plants and animals is their cell structure. Plant cells have cell walls, animals do not.

In a gross, or large scale sense,

plants mostly create their own food while all animals must consume external sources of energy such as plants or other animals;.

animals usually have a way to move on their own, where plants do not;

animals usually react quickly to external events, whereas plants do react, but often more slowly.

In all ways, jellyfish qualify as animals.

The existence of a heart and identifiable brain are not requirements for being an animal. (Boy, does that open things up for a lot of wise cracks!)

Hope this helps. R. W. "Mr. A." Avakian Instructor Arts and Sciences/LRC Oklahoma State Univ. Inst. of Technology

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