Hydra Unlabelled Diagrams – Whole Hydra and Stinging Cell

All 14 parts on the diagram

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The hydra 9

Tentacles A ring of long hollow arms. They reach out, catch small water animals with their stinging cells and carry them to the mouth.
Mouth The only opening of the body. Food goes in through it, and what cannot be digested comes back out the same way.
Hypostome The cone at the top of the body, inside the tentacle ring. The mouth is at its tip.
Gastrovascular cavity The 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.
Ectoderm The outer layer of cells. It protects the animal and senses the water round it, and the stinging cells lie in it.
Endoderm The 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.
Mesoglea A very thin layer of jelly between the two layers of cells.
Bud A young hydra growing from the side of its parent. It forms its own tentacles and mouth, then breaks away to live alone.
Basal disc The 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

Cnidoblast The cell that makes and holds one stinging capsule. Newer books call it a cnidocyte. They are most numerous on the tentacles.
Nematocyst The stinging capsule, a tough pear-shaped bag of fluid. A long hollow thread is coiled inside it.
Cnidocil A trigger hair that stands out from the cell. When prey touches it, the capsule fires.
Thread A long hollow thread. It lies coiled in the capsule and turns inside out as it shoots out.
Barbs Small 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

Two hydra diagrams on one sheet, both blank: the animal cut lengthwise on its twig, and under it a single stinging cell drawn before and after it fires. Fourteen lines run out from the parts, each waiting for a name.

Read the full guide

Hydra diagram unlabeled: the word list

Nine names go on the hydra: tentacles, mouth, hypostome, gastrovascular cavity, ectoderm, endoderm, mesoglea, bud and basal disc. Five go on the stinging cell: cnidoblast, nematocyst, cnidocil, thread and barbs.

Three lines into the body wall

Three lines on the left of the body end close together. The top one stops on the thin line between the two bands: mesoglea. The middle one stops on the outer band, the ectoderm, and the lowest crosses to the inner band, the endoderm.

Two lines reach the top of the body. One stops at the very tip, the mouth; the other on the cone below, the hypostome.
The stinging cell is drawn twice. Label the cnidocil, nematocyst and cnidoblast on the left one, and the thread and barbs on the fired one.
A newer textbook may print epidermis for ectoderm, gastrodermis for endoderm and cnidocyte for cnidoblast.

Questions and answers

What are the parts of a hydra to label?

Fourteen on this sheet: nine on the animal, from the tentacles to the basal disc, and five on its stinging cell. The twig, the dashed lines and the nucleus of the cell are drawn but take no name.

What is the difference between a cnidoblast and a nematocyst?

The cnidoblast is the whole stinging cell. The nematocyst is the capsule inside it, the part that holds the coiled thread. On the sheet one line ends in the outer part of the left cell and another on the capsule within.

What is the hollow inside a hydra called?

The gastrovascular cavity, the one space inside the body. Its line comes in from the right and ends in the open middle of the cut body, low down. Older books call it the enteron.

Which part of the hydra diagram is the bud?

The small branch on the right side of the body, with short arms of its own. It is a young hydra still joined to its parent, and its line ends inside it, not on the parent's wall.