Body plan of a polyp (A) and a medusa (B), showing their similarity. In red, the ectoderm; in blue, the endoderm; in black, the mesoglea ; in yellow, the gastric cavity which opens to the outside through the mouth. Modified after Pearse et al. “Living Invertebrates”, 1987. See the animation

Tissues and cell types

The ectoderm contains epithelio-muscular cells (EMC) which are in contact with the mesoglea. Between those cells are found stinging cells also called nematocytes, multipotent interstitial cells, nervous cells, sensory cells, and gland cells (which produce the external skeleton or perisarc). The epithelio-muscular cells contain smooth muscle fibers which are arranged lengthwise. They allow polyp extension or contraction. The endoderm is principally composed of epithelio-muscular cells in touch with the mesoglea, digestive cells, and glandular cells which are involved in digestion. Those cells have flagella whose beating induces a flow in the gastrovascular cavity. The endoderm also contains nervous and sensory cells. The smooth muscle fibers of the epithelio-muscular cells are circular. They allow polyp constriction or dilatation and their action is thus antagonist to the action of lengthwise ectodermic muscle fibers.

Lengthwise and cross section of a polyp (modified after Pearse et al.).

Nematocysts

Nematocytes - also called cnidocytes - are stinging cells characteristic of the Cnidaria: they are found exclusively within the animals of this Phylum.

Each nematocyte contains a very particular structure which is a capsule produced by the cell itself and called nematocyst or cnidocyst. The functioning of the nematocyst is triggered thanks to a sensitive structure, the cnidocil, present at the surface of the cell. The nematocysts are classified into different types according to their microscopic structure and their function. Most of them are involved in prey capture and neutralization. In the penetrating (piercing) type (illustrated in the opposite drawing with a stenotele), the capsules contain a toxic liquid in which a filament, provided with thorns, is immersed. Under the effect of a stimulus perceived by the cnidocil (e.g. contact with a living organism), the filament is launched to the outside just like a tiny harpoon equipped with a syringe. In the volvent type, once extruded, the filament which is devoid of thorns winds round the target (e.g.desmonemes). In the Hydrozoans, stinging cells are abundant in the tentacles of polyps and medusae. They may be evenly scattered on the surface of the tentacles, or gathered in clusters of different size and shape. They are also present in the umbrella of some medusae (see hereafter Polyp and Medusa morphology in S. nipponica).

Discharge of a penetrating nematocyst (stenotele) after Ruppert & Barnes.

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2. Polyp morphology

The larger part of the polyp body is called the column. At the tip of it, the hypostome represents the oral part. The polyp bears tentacles and contains a gastrovascular cavity. It is supported at its base by a pedicel.

		(a) The hypostome is dome-like shaped when at rest, but is capable of distorting when in activity. It bears the mouth at its apex. (b) The column comprises two distinct parts: a digestive (distal) part where the digestive and gland cells are concentrated, and a pedal (basal) part with a thick supporting ectoderm. Within the reproduction period, medusa budding occurs on the column (see Reproduction). (c) The pedicel The pedicel is the structure connecting the polyp to the rest of the colony. It is composed of a tissue (the perisarc).
(A) Diagrammatic lengthwise cross of a polyp (after Grassé, Traité de Zoologie, 1994) : (a) the hypostome (b) the column and (c) the sphincter. 1. The oral aperture 2. tentacles 3. medusa bud 4. sphincter 5. perisarc 6. pedicel. (B) Photograph of a living polyp of S. nipponica.

Tentacles

There are two types of tentacles in S. nipponica : capitate and acnid. The hypostome is surrounded by a whorl of 4-5 oral capitate tentacles of great size. Some other smaller capitate tentacles are present along the column in various number. A ring of 4-5 acnid tentacles is sometimes present at the base of the polyps. They have sensory cells bearing long filaments (stereocils). In Hydroids, stinging cells are not necessarily concentrated in a unique terminal cluster as they are in the capitate tentacles. The arrangement of those cells in tentacles determines their type.

Capitate tentacle (TC) and sensory acnid tentacle (TS)	Oral ring of capitate tentacles	Contracted acnid tentacle bearing long sensory filaments (stereocils)

 

Nematocysts

			In S. nipponica the nematocytes are gathered in the terminal capitations of the tentacles of the polyps. The nematocysts contained in the cells are clearly visible under compound microscope because of their refringence. In the opposite photographs, stenotele nematocysts of various size can be seen. In the right photograph a discharged stenotele is visible with the basal (proximal) part of its filament.
Terminal capitation of a tentacle of a polyp	Undischarged nematocysts	Discharged nematocyst

Colonies of polyps

In many hydroid species, polyps are not solitary but they build colonies composed of tens and even millions of them. The polyps of a same colony remain connected to each other through a tubular tissue (the coenosarc), protected by a tubular skeleton (the perisarc), forming a network.

Protective perisarc surrounding the stolons and the base of the polyps (red arrows), forming pseudohydrothecae (here the skeleton is devoid of living tissues).

On Reunion Island, colonies of S. nipponica only comprise a few dozen polyps. They are stolonal and rambling. The chitinous skeleton that surrounds the stolons extends to the base of each polyp, forming what is called "pseudohydrothecae" (as opposed to the "true" hydrothecae). That particular morphological feature leads S. nipponica to be placed in the Sub-Class Athecates hydroid species while true hydrothecae are placed within the Thécates.

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3. Medusa morphology

Diagrammatic structure of an hydromedusa (after Pearse et al., Living Invertebrates, 1987)

The medusa is principally composed of a translucent bell called umbrella, whose mesoglea is thick. The size of the umbrella determines the size of the medusa: about 1 mm for S. nipponica. The shape of the umbrella depends on the species. The convex (external) face of the umbrella (the exumbrella), is directed upwards, whereas the concave (internal) face (the subumbrella) is downward leaning and delimits the subumbrellar cavity.

The gastrovascular system

The organ that hangs in the subumbrellar cavity is called the manubrium, and is equivalent to the stomach of other animals. The mouth is situated at the free end of it. From the other end run four radial canals that open into a circular canal surrounding the base of the umbrella at the edge of the velum. The inner wall of these canals is composed of endodermic cells with flagelli (like the endodermic cells of the polyp stage). This communicating canal system is the gastrovascular system dof the medusa: it allows nutritive particles and gazes to be distributed in a solution throughout all parts of the medusa body. Sexual maturation of the medusae of S. nipponica occurs with the development of gonads on the manubrium, a characteristic feature that places those medusae within the Anthomedusa) (see chapter Reproduction).

Position of radial and circular canals

Velum

Hydromedusae are craspedote medusae, i.e. they have an ectodermic membrane called velum that makes the aperture of the subumbrellar cavity narrower. The velar aperture plays a role in locomotion (see Behaviour).

Velum and velar aperture	Velum (red arrow), circular canal, tentacular bulbs and tentacles (seen in oral view = from below)

Tentacles

The medusae of S. nipponica have 4 radial tentacles lined up with the radial canals. Unlike the tentacles of polyps, those of medusae are hollow and contain the gastrovascular cavity extension.

		Nematocyte clusters, which are involved in the capture of prey, as well as defence, are unevenly scattered along the tentacles. Moreover, there is a flat sticky pad at the end of the tentacles that allow medusae to cling to a support for a while (GB et Flammang, unpublished results, see Behaviour).
Clusters of nematocysts on tentacles	End of a tentacle seen under electron microscope

Ocelli

	Four structures, the tentacular bulbs, are found at the intersection between the tentacles and the umbrellar margin, where the 4 radial canals fall into the circular canal. In S. nipponica, the tentacular bulbs bear sensory organs, the ocelli, which are photosensitive pigment spots/photoreceptors.
An ocellus seen on the outside of the tentacular bulb

Nematocysts

	Nematocysts are numerous and gathered in clusters on tentacles. They also form a ring around the mouth. In young medusae (which have just separated from the polyps), they are found in the ectoderm of the exumbrella but disappear as the medusae grow and come to sexual maturity. The two types of nematocysts found in medusae are stenoteles and desmonemes.
Spent nematocysts (stenoteles) found in tentacles