TentaclesA ring of long hollow arms. They reach out, catch small water animals with their stinging cells and carry them to the mouth.
MouthThe only opening of the body. Food goes in through it, and what cannot be digested comes back out the same way.
HypostomeThe cone at the top of the body, inside the tentacle ring. The mouth is at its tip.
Gastrovascular cavityThe single hollow inside the body. It runs from the mouth to the foot and out into each tentacle, and the food is digested in it.
EctodermThe outer layer of cells. It protects the animal and senses the water round it, and the stinging cells lie in it.
EndodermThe inner layer of cells. It lines the cavity and digests the food. It is about twice as thick as the outer layer. Older books spell it entoderm.
MesogleaA very thin layer of jelly between the two layers of cells.
BudA young hydra growing from the side of its parent. It forms its own tentacles and mouth, then breaks away to live alone.
Basal discThe flat foot, with gland cells that make a sticky fluid. The fluid holds the hydra to a stone or a water plant.
A stinging cell 5
CnidoblastThe cell that makes and holds one stinging capsule. Newer books call it a cnidocyte. They are most numerous on the tentacles.
NematocystThe stinging capsule, a tough pear-shaped bag of fluid. A long hollow thread is coiled inside it.
CnidocilA trigger hair that stands out from the cell. When prey touches it, the capsule fires.
ThreadA long hollow thread. It lies coiled in the capsule and turns inside out as it shoots out.
BarbsSmall sharp points at the base of the thread.
How this list was compiled: Every name checked on 2026-09-30 in two sources. R. W. Hegner, College Zoology (1912, public domain), https://archive.org/details/collegezoology00hegn. T. J. Parker and W. A. Haswell, A Text-book of Zoology (1910, public domain), https://archive.org/details/textbookofzoolog00park. Wikipedia, Hydra (genus) and Cnidocyte, https://en.wikipedia.org/wiki/Hydra_(genus). Animal Diversity Web, University of Michigan Museum of Zoology, Cnidaria, https://animaldiversity.org/accounts/Cnidaria/. Licence: own work
The drawing: Drawn for this site on 2026-10-05 by script. Our own drawing, nothing traced. The habit of the animal (slender column, long beaded tentacles, a bud low on the column, a spreading foot) was laid out by eye after the figure of Hydra with a bud in V. L. Kellogg, The Animals and Man (1911), public domain (https://commons.wikimedia.org/wiki/File:The_animals_and_man;_an_elementary_textbook_of_zoology_and_human_physiology_(1911)_(14598161549).jpg); the scheme of the lengthwise section after D. and A. N. McAlpine, Biological Atlas (1880), plate X, public domain (https://commons.wikimedia.org/wiki/File:Biological_atlas_(Plate_X)_(6441474303).jpg); the stinging cell after the figure of nematocysts, after Iwanzov, in Encyclopaedia Britannica (1911), article Hydromedusae, public domain (https://commons.wikimedia.org/wiki/File:EB1911_Hydromedusae_-_structure_of_Nematocysts_and_their_mode_of_working.jpg). A plan, much enlarged and not to scale: the body wall is drawn far thicker than it is so its layers show, only two of the six tentacles are shown cut, the lowest pair of tentacles leaves the body a little below the ring of the others, and the thread is drawn far shorter than it is. Licence: own work
How it works
The parts of a hydra fit in a very small body: a hollow tube with a mouth at one end and a sticky foot at the other. The first view cuts it lengthwise. The second enlarges one stinging cell.
Read the full guide
The hydra
Tentacles ring the hypostome, the cone that carries the mouth. The wall of the body has two layers of cells, ectoderm outside and endoderm inside, with thin mesoglea between them. A bud grows from the side, and the basal disc holds the animal down.
A stinging cell
Switch to Stinging cell. Each cnidoblast holds a nematocyst with a thread coiled inside. A touch on the cnidocil fires it: the thread turns inside out and shoots out, with barbs at its base. Press Quiz me to name each part yourself.
The body wall is drawn far thicker than it is, so that its two layers and the mesoglea between them can be told apart.
Questions and answers
How does a hydra eat?
Its tentacles catch small water animals with their stinging cells and carry them to the mouth. The food is digested in the gastrovascular cavity, and what is left goes back out through the mouth.
How does a hydra reproduce?
Most often by budding. A bud grows from the side of the body, forms its own tentacles and mouth, and breaks away as a new hydra.
Is a hydra an animal?
Yes. A hydra is a small freshwater animal, a relative of jellyfish and corals. It has no brain, only a net of nerve cells.
What are a hydra's stinging cells called?
Cnidoblasts, or cnidocytes. Each holds one stinging capsule, the nematocyst. They are found on every part of the body except the basal disc, and most of all on the tentacles.