Illustrated Glossary of Sea Anemone Anatomy - Terms

This page contains a list of terms and definitions relevant to sea anemones and their anatomy. The definitions in this glossary are largely adapted from the glossaries of terms in Dr. Oskar Carlgren's "A Survey of the Ptychodactiaria, Corallimorpharia and Actiniaria" (1949) and Dr. T. A. Stephenson's "Coelenterata" (1918).

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Acontia: thin white or colored threads attached at one end to the borders of the mesenteries, as a rule below the filaments, while the other end is free. They are laden with extraordinarily numerous nematocysts of various categories. They can be protruded through the mouth, and in some cases through special pores (cinclides) in the body-wall, for purposes of defense or paralyses of prey. In histological structure they differ completely from the mesenterial filaments.

Acontioids: thick threads attached to mesenteries below the filaments. With few nematocysts belonging to some of the same categories as those in the filaments.

Acrorhagi: marginal outgrowths of the body-wall found in some genera of Actiniaria, containing a strong concentration of nematocysts. They may be simple and spherical, slightly branched, or even frondose. Follow this link to find images of acrorhagi.

Acrospheres: the globular ends of certain tentacles, laden with numerous nematocysts. They are of different categories in the Corallimorpharia and in the Actiniaria and seemingly in Madreporaria also.

Actinopharynx: throat, stomodaeum (or gullet, aesophagus, or pharynx); the tube which leads from the mouth in to the coelenteron. Follow this link to find images of actinopharynges.

Actinostome: the mouth, or upper aperture of the actinopharynx, in the center of the oral disk.

Alveolar: in cross section the mesogloeal sphincters of many anemones are composed of numerous small more of less circular groups of muscle fibers, these small circles having the appearance of alveoli. In a sphincter in which the "alveoli" are fairly scattered, the structure may be termed "alveolar". When the "alveoli" are so close together that the strands of mesogloea between them are very reduced, and form a network, the structure may be termed "reticular".

Atrichs: see Nematocysts.


Basilar muscles: see Mesenterial muscles.

Basitrichs: see Nematocysts.


Capitulum: see Scapus.

Ciliated tracts: see Filament.

Cinclides: small apertures (or organized soft spots which will rupture readily) in the column through which the acontia, if present, may protrude. Follow this link to find images of cinclides.

Circumscribed retractor: see Mesenterial muscles.

Circumscribed sphincter: see Sphincter.

Cnidae: the same as nematocysts.

Cnido-glandular tract: see Filament.

Coelenteron: the general cavity of the body of an anemone where digestion, nutrient absorption, and gas exchange take place.

Collar: see Fosse.

Column: body-wall.

Conchula: a more or less lobed projection from the upper end of a siphonoglyph.

Couple: the 12 oldest mesenteries originate in a different way from those which follow in that they arise bilaterally on opposite sides of the directive axis. Each such "pair" has been called a "couple". The directives, which normally belong to the 12 primary mesenteries, are couples as well as pairs. Follow this link to find images of couples and other mesenteries.


Diffuse retractor: see Filament.

Diffuse sphincter: see Sphincter.

Diploblastic: having a body made of two cellular layers only (as opposed to the majority of animals which are triploblastic), the exterior ectoderm and the interior endoderm. These two layers are separated by the largely non-cellular jelly-like mesogloea. Follow this link to find images showing the diploblastic nature of the column and other features.

Directives: see Mesenterial arrangement.

Directive axis: see Orientation of the body.

Dorsal and Dorso-lateral: see Orientation of the body.


Ecto-mesogloeal muscles: muscles which in the main are embedded in the mesogloea.

Endocoel: the space between two mesenteries belonging to one and the same pair.

Endocoelic tentacles: tentacles the cavities of which communicate with endocoels.

Endo-mesogloeal muscles: muscles which in the main are embedded in the mesogloea.

Enterostome: the lower or internal orifice of the actinopharynx, by which the cavity of the latter is put into communication with the coelenteron.

Exocoel: the space between mesenteries belonging to different pairs.

Exocoelic tentacles: tentacles the cavities of which communicate with exocoels.


Filament: a thickened rim running along the free border of a mesentery from the end of the actinopharynx (in the case of "perfect mesenteries") downwards. In its lower part the filament is simple, in its upper part it may be a triple cord. The lateral bands have been called ciliated tracts (streaks), the median band the cnidoglandular tract (streak) and this may be very convoluted below the ciliated tracts. In the Corallimorpharia and Protantheae the filament is always simple, in the Ptychodactiaria the simple filament is continued distally into a half-funnel formation. Follow this link to find images of filaments and mesenteries.

Fosse: a circular groove enclosed by a distinct fold, the parapet or collar, of the column, a little below the tentacles.


Holotrichs: see Nematocysts.

Hoplotelic: see Nematocysts.


Imperfect mesenteries: mesenteries not reaching the actinopharynx. Follow this link to find images of imperfect mesenteries.


Limbus: the border along which the column joins the pedal disk.


Macrobasic amastigophors: see Nematocysts.

Macrobasic mastigophors: see Nematocysts.

Macrocnemes: see Mesenterial arrangement.

Margin: the upper edge of the column, just below the roots of the tentacles.

Marginal spherules: vesicles situated on the parapet or in the fosse, which may have apertures, and are provided with atrichs, basitrichs and spirocysts. Follow this link to find images of marginal spherules.

Marginal stomata: large or small apertures in large mesenteries (one aperture per mesentery), near the margin of the body, by means of which water may pass through the mesenteries.

Marginal pseudospherules: vesicles situated at the margin, which may possess an aperture, and containing basitrichs only. Follow this link to find images of marginal pseudospherules.

Mesenteries: infoldings of the endoderm and mesoglea extending from the body-wall into the gastrovascular cavity, some of which reach the actinopharynx and divide the gastrovascular cavity. The mesenteries serve to increase the surface area for digestion and uptake of nutrients. Follow this link to find images of mesenteries.

Mesenterial arrangement: The mesenteries are arranged in pairs, each consisting of two mesenteries adjacent to one another. One can distinguishes between directive pairs and ordinary pairs. Directives, which are situated in the directive axis, have their longitudinal muscles (retractors) on their outer sides turned towards the exocoels. In ordinary pairs, the longitudinal muscles (retractors) are situated on their inner sides, turned towards the endocoels. The directives are exclusively perfect, the other pairs may be perfect or imperfect. For the most part, the partners of the ordinary pairs are equally developed, but they may be very different in size. The arrangement of the pairs can be six-rayed (hexamerous), eight-rayed (octamerous), or ten-rayed (decamerous). Irregularities in the arrangement are, however fairly common in connection with displacement or absence of the directives or dislocation of tentacles. In elongate forms the perfect mesenteries may be strongly differentiated from the imperfect ones. The former, macrocnemes, have very strong retractors, gonads and filaments, the latter, microcnemes, lack these organs. Intermediates may, however, occur. The arrangement of mesenteries may be quite different from the type, as in the Minyadidae, Endocoelantheae, Exocoelactiidae and some Actinostolidae. Follow this link to find images of the arrangement of mesenteries.

Mesenterial muscles: One side of each mesentery is occupied by longitudinal muscles, the other by transverse and parietobasilar muscles; the latter generally run obliquely from the column to the pedal disc. In most of the Actiniaria there are also basilar muscles running along both sides of the base of the mesentery, close to the pedal disc. The longitudinal muscles are generally more or less concentrated, forming more or less strong retractors, of different appearance in cross section. When the muscles are very strongly concentrated and there is only one mesogloeal lamella (or a few main lamellae close to each other) issuing from the main lamella of the mesentery one speaks of a circumscript retractor. Restricted retractors show about the same degree of concentration, but in this case a number of more scattered muscle lamellae arise from the mesentery. If the retractor is not strongly concentrated we call it diffuse. It may occur, especially in elongate Athenaria, that the longitudinal muscles are very weak apart from the retractor. Close to the body-wall they increase in size and form, together with the parietobasilar muscles (which here run longitudinally) a parietal muscle. Follow these links to find images of parietal muscles, retractor muscles, and sphincter muscles.

Meso-ectodermal muscles: muscles which in the main are ectodermal, but small parts of which are embedded in the mesogloea.

Mesogloeal muscles: muscles wholly embedded in the mesogloea.

Meso-endodermal muscles: muscles which in the main are endodermal, but a small part of which is embedded in the mesogloea.

Metacnemes: the mesenteries arising as pairs after the formation of the 12 first mesenteries. Follow this link to find images of metacnemes.

Microbasic b-mastigophors: see Nematocysts.

Microbasic p-mastigophors: see Nematocysts.

Microcnemes: see Mesenterial arrangement.


Nematoblasts: special cells in which nematocysts are housed.

Nematocysts: stinging capsules the thread of which shows several types of structure. The following categories of nematocysts are present in the Anthozoa: atrichs: thread without barbs, smooth; holotrichs: thread without a differentiated basal shaft but with barbs along its whole length: basitrichs: thread without shaft but with barbs at its base only; microbasic b-mastigophors: thread with a shaft, but the demarcation between the shaft and the thread not strongly marked, shaft with barbs; in unexploded capsules the shaft does not show any funnel-shaped formation; microbasic p-mastigophors: demarcation between the shaft and the thread strongly marked, shaft with barbs; a funnel-shaped formation in the shaft at the beginning of the distal part of the thread is visible in unexploded capsules; the thread of the microbasic mastigophors may be armed: then it is termed hoplotelic; microbasic amastigophors: the thread reduced, only the shaft present, at most 3 times as long as the capsule, shaft with barbs, a funnel-shaped formation visible at the end of the shaft in unexploded capsules; macrobasic amastigophors: as in the former, but the shaft more than 3 times as long as the capsule; in the unexploded capsule the shaft forms coils. Follow this link to find images of nematocysts.

Nematosomes: globular strongly ciliated, free swimming bodies with numerous nematocysts occurring in the coelenteric cavity of Nematostella. Their true nature is unknown.

Nematospheres: globular tentacles with numerous basitrichs. Follow this link to find images of nematospheres.

Nemathybomes: spheroid invaginations of the columnar ectoderm into the mesogloea laden with numerous nematocysts. They occur in Edwardsia and Isoedwardsia. Follow this link to find images of nemathybomes.


Orientation of the body: A typical animal belonging to the groups handled here can be divided into two equivalent halves by a line passing through the endocoels between the directive mesenteries. This line is the directive axis. In order to describe the position of the 8 primary mesenteries one uses the arbitrary terms dorsal and ventral for the two directive couples and dorso-lateral and ventro-lateral for the lateral ones. The ventral directive couple is the one towards which the retractors on the four lateral mesenteries face.


Palmate sphincter: see Sphincter.

Parapet: see Fosse.

Parietal muscles: see Mesenterial muscles.

Parietobasilar muscles: see Mesenterial muscles.

Perfect mesenteries: mesenteries attached to the actinopharynx. Follow this link to find images of perfect mesenteries.

Physa: the aboral ampullaccous end of certain Athenaria, appears rounded and swollen out into a bladder-like structure. Follow this link to find images of physae.

Pinnate sphincter: see Sphincter.

Primary tentacles: the six oldest tentacles. Follow this link to find images of primary tentacles.

Primary mesenteries: the six oldest pairs of mesenteries. Follow this link to find images of primary mesenteries.

Protocnemes: the first 12 mesenteries which arise as couples. Follow this link to find images of protocnemes.


Restricted sphincter: see Sphincter.

Restricted retractor: see Mesenterial muscles.

Retractors: see Mesenterial muscles.


Scapus: In some taxa the column is externally divisible into regions. The most proximal zone has been called a physa but this is an ampullaccous extremity present only in Athenaria. The principal and longest zone of the column or scapus may be provided with tenaculi or tubercles. Above the scapus, distally, there is either a thick-walled scapulus or a thin-walled capitulum. In certain cases both regions are present, the capitulum above the scapulus. Follow this link to find images of the regions of the column.

Scapulus: see Scapus.

Siphonoglyphs: Anatomically differentiated smooth grooves running down the actinopharynx from the mouth to its inner end or beyond this. For the most part they are connected with directives but not in all cases. In some taxa the siphonoglyph forms a tube separated from the actinopharynx (in Peachia mira and Metapeachia). Follow this link to find images of siphonoglyphs.

Sphincter: The endodermal circular muscles of the column may be accumulated at or near the margin and form a sphincter which either is endodermal or embedded in the mesogloea, which is then called a mesogloeal sphincter. Rarely there is a transition between them, an endo-mesodermal or meso-endodermal sphincter. The endodermal sphincter shows a different appearance in cross section. If it is elongate and broadly attached to the column we speak of a diffuse sphincter, if more concentrated a restricted one. The most concentrated endodermal sphincter is the circumscribed type within which we can distinguish two kinds, a pinnate with only one main-lamella and a palmate with only a few main-lamellae. There may also be two mesogloeal sphincters in the same animal. In the genus Bolocera and some allied genera, and in the family Boloceroididae, the base of each tentacle is provided with an endodermal sphincter. A weak endodermal sphincter may also be present to close the cinclides. Follow this link to find images of sphincter muscles.

Suckers: see Verrucae.


Tenaculi: more or less solid papillae situated on the column, the ectoderm of which is partly chitinised and provided with an usually strong, sometimes stratified cuticle, to which grains of sand or detritus may adhere. Follow this link to find images of tenaculi.


Ventral and ventro-lateral: see Orientation of the body.

Vesicles: ampullaccous, non-adhesive evaginations of the column, simple or compound; with more or less numerous nematocysts of various categories. Follow this link to find images of vesicles.

Verrucae: more or less ampullaccous, adhesive evaginations of the column, simple or more rarely compound, with modified ectoderm, without nematocysts in their central part. Rarely, as in Sagartia, there is no evagination, but the ectoderm shows same structure as that of the verrucae proper and is adhesive (suckers). Follow this link to find images of verrucae.

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