noun
definition
An abnormal growth protruding from a mucous membrane
definition
A cylindrical coelenterate, such as the hydra, having a mouth surrounded with tentacles
Examples of polyp in a Sentence
The coral polyp is also found in Venezuelan waters.
In the polyp the nervous tissue is always in the form of a scattered plexus, never concentrated to form a definite nervous system as in the medusa.
F, the founder-polyp; I, 2, 3, 4, the succession of polyps budded from the founder-polyp; a', b', c', the succession of polyps budded from 1; a 2, 2 polyps budded from 2; a 3, polyp budded from 3.
Or a polyp on the main stem, after having budded a second time to form a pinnule, may give rise to a third bud, which starts a new biserial FIG.
The histology described above for the polyp may be taken as the primitive type, from which that From Allman's G y mnoblastic Hydroids, by permission of the Council of the Ray Society.
The muscle-fibres arise as processes from the bases of the epithelial cells; such cells may individually become sub-epithelial in position, as in the polyp; or, in places where muscular tissue is greatly developed, as in the velum or sub-umbrella, the entire muscular epithelium may be thrown into folds in order to increase its surface, so that a deeper sub-epithelial muscular layer becomes separated completely from a more superficial bodyepithelium.
In contrast with the polyp, the longitudinal muscle-system is entirely ectodermal, there being no endodermal muscles in craspedote medusae.
Moreover, all the medusae budded from a given hydroid colony are either male or female, so that even the non-sexual polyp must be considered to have a latent sex.
The genus Myriothela is a solitary polyp with scattered capitate tentacles, producing sporosacs.
Pelagohydridae, for the floating polyp Pelagohydra, Dendy, from New Zealand.
The animal is a solitary polyp bearing a great number of medusa-buds.
Have a nasal polyp all each one startling reminder of license to harrah's.
In Ieptolinae the embryonic development culminates in a polyp, which is usually formed by fixation of a planula (parenchymula), rarely by fixation of an actinula.
The polyp is regarded, on this view, as a form phylogenetically older than the medusa, in short, as nothing more than a sessile actinula.
The whole theory is one most intimately connected with the question of the relation between polyp and medusa, to be discussed presently.
For the most part, polyp and medusa have been regarded as modifications of a common type, a view supported by the existence, among Scyphomedusae (q.v.), of sessile polyp-like medusae (Lucernaria, &c.).
In some polyps the tentacles are webbed at the base, and it was supposed that a medusa was a polyp of this kind set free, the umbrella being a greatly developed web or membrane extending between the tentacles.
Mechnikov considered the plate thus formed at the base of the polyp as equivalent to the umbrella, and the body of the polyp as equivalent to the manubrium, of the medusa; on this view the marginal tentacles almost invariably present in medusae are new formations, and the tentacles of the polyp are represented in the medusa by the oral arms which may occur round the mouth, and which sometimes, e.g.
The question is one intimately connected with the view taken as to the nature and individuality of polyp, medusa and gonophore respectively.
The theory that the medusa is an independent individual, fully equivalent to the polyp in this respect, is now universally accepted as being supported by all the facts of comparative morphology and development.
Balfour put forward the view that the polyp was the more primitive type, and that the medusa is a special modification of the polyp for reproductive purposes, the result of division of labour in a polypcolony, whereby special reproductive persons become detached and acquire organs of locomotion for spreading the species.
Brooks, on the other hand, as stated above, regards the medusa as the older type and looks upon both polyp and medusa, in the Hydromedusae, as derived from a free-swimming or floating actinula, the polyp being thus merely a fixed nutritive stage, possessing secondarily acquired powers of multiplication by budding.
The Hertwigs when they discovered the endoderm-lamella showed on morphological grounds that polyp and medusa are independent types, each produced by modification in different directions of a more primitive type represented in development by the actinulastage.
If a polyp, such as Hydra, be regarded simply as a sessile actinula, we must certainly consider the polyp to be the older type, and it may be pointed out that in the Anthozoa only polyp-individuals occur.
This must not be taken to mean, however, that the medusa is derived from a sessile polyp; it must be regarded as a direct modification of the more ancient free actinula form, without primitively any intervening polyp-stage, such as has been introduced secondarily into the development of the Leptolinae and represents 'a revival, so to speak, of an ancestral form or larval stage, which has taken on a special role in the economy of the species.
The body bears tentacles, but shows no division into hydrorhiza, hydrocaulus or hydranth; it is temporarily fixed and has no perisarc. The polyp is usually hermaphrodite, developing both ovaries and testes in the same individual.
In Hydra viridis the polyp is of a green colour and produces a spherical egg with a smooth shell which is dropped into the mud.
The polyp may be solitary, but more usually produces polyps by budding and forms a polyp-colony.
The gymnoblastic polyp usually has a distinct perisarc investing the hydrorhiza and the hydrocaulus, sometimes also the hydranth as far as the bases of the tentacles (Bimeria); but in such cases the perisarc forms a closely-fitting investment or cuticule on the hydranth, never a hydrotheca standing off from it, as in the next sub-order.
In some cases, any polyp of the colony may bud medusae; in other cases, only certain polyps, the blastostyles, have this power.
Nemopsidae, for the floating polyp Nemopsis, very similar to Tubularia in character; the medusa, on the other hand, is very similar to Hippocrene (Margelidae).
The calyptoblastic polyp of the nutritive type is very uniform in character, its tendency to variation being limited, as it were, by the enclosing hydrotheca.
This is in some degree parallel to the cases described above, in which a planula gives rise to the hydrorhiza, and buds a polyp laterally.
In this connexion must be mentioned, finally, the medusae budded from the fresh-water polyp Microhydra.
In both cases the hydranth is extremely reduced and has no tentacles, and the polyp forms a colony by budding from the base.
In view of the great resemblance between Microhydra and the polyp of Limnocodium, it might be expected that the medusae to which they give origin would also be similar.
Huxley, therefore, considered a hydroid colony, for example, as a single individual, and each separate polyp or medusa budded from it as having the value of an organ and not of an individual.
Thus a bract may be regarded, with Haeckel, as a modified umbrella of a medusa, a siphon as its manubrium, and a tentacle as representing a medusan tentacle shifted in attachment from the margin to the sub-umbrella; or a siphon may be compared with a polyp, of which the single tentacle has become shifted so as to be attached to the coenosarc and so on.
In the first place, we find in this group two distinct types of person or individual, the polyp and the medusa (qq.v.), each capable of a wide range of variations; and when both polyp and medusa occur in the life-cycle of the same species, as is frequently the case, the result is an alternation of generations of a type peculiarly characteristic of the class.
The morphology of the Hydrozoa reduces itself, therefore, to a consideration of the morphology of the polyp, of the medusa and of the colony.
Putting aside the last-named, for a detailed account of which see Hydromedusae, we can best deal with the peculiarities of the polyp and medusa from a developmental point of view.
The gastrula has now become an actinula, which may be termed the distinctive larva of the Cnidaria, and doubtless represents in a transitory manner the common ancestor of the group. In no case known, however, does the actinula become the adult, sexually mature individual, but always undergoes further modifications, whereby it develops into either a polyp or a medusa.
It is convenient to distinguish two types of polyp by the names hydro polyp and anthopolyp, characteristic of the Hydrozoa and From Gegenbaur's FIG.
Thus the body becomes umbrellashaped, the concave side representing the peristome, and the convex side the column, of the polyp. Hence the tentacles are found at the edge of the umbrella, and the hypostome forms usually a projecting tube, with the mouth at the extremity, forming the manubrium or handle of the umbrella.
It is sufficient to state here that the medusa is usually a free-swimming animal, floating mouth downwards on the open seas, but in some cases it may be attached by its aboral pole, like a polyp, to some firm basis, either temporarily or permanently.
On the other hand, the parenchymula may develop directly into the actinula or even into the polyp, with suppression of the intervening steps.
In Cordylophora the embryo is set free at the parenchymula stage as a planula which fixes itself and develops into a polyp, both gastrula and actinula stages being suppressed.
In Tubularia, on the other hand, the parenchymula develops into an actinula within the maternal tissues, and is then set free, creeps about for a time, and after fixing itself, changes into a polyp; hence in this case the planula-stage, as a free larva, is entirely suppressed.
The Hydrozoa may be defined, therefore, as Cnidaria in which two types of individual, the polyp and the medusa, may be present, each type developed along divergent lines from the primitive actinula form.
The polyp (hydropolyp) is of simple structure, and never has an ectodermal oesophagus or mesenteries.