POLYMORPHISM In the articles COELENTERATA and ANTHOZOA, and in accounts of the groups of Hydrozoa already dealt with in the present article, references will be found to the phenomena known as polymorphism and alternation of generations (metagenesis). Since these phenomena are aspects of one and the same thing, and are of general interest and importance, a summary of the subject is indicated. In the following remarks it is taken for granted that the articles above mentioned are familiar to the reader.
Firstly it must be made clear that alternation of generations as found in Obelia is simply an example of polymorphism, which may be defined as the ability of a single species of animal to exist under more than one form—in this instance as polyp and medusa. In a clear-cut case of alternation of generations one of these forms is sexual and succeeds the alternative non-sexual form, the two co-existing on a single colony only for the period during which the medusae are developing from buds.
The other type of polymorphism is that which is exhibited by such colonies as Hydractinia and in far higher degree by the Siphonophora, and in which there is not only the distinction of individuals into sexual and non-sexual forms, but the non-sexual polyps are themselves divided into kinds (gastrozooids, dactylo zooids, etc.). This is exemplified even in Obelia by the distinction of the polyps into hydranths and blastostyles. In the Siphono phora there is the additional development that here there exist non-sexual as well as sexual medusae, and probably more than one kind of these (swimming-bells, bracts and float).
In other words there exists (a) a differentiation into sexual and non-sexual forms and (b) a division of labour between the non sexual forms by virtue of which they become, for practical pur poses, reduced to the condition of organs.
It must be noted however that the sexual forms tend to lose their independence ; and the separate free-swimming generation so obvious in Obelia and similar forms becomes at the other end of the series a degenerate sporosac attached to the colony as permanently as any non-sexual individual. In fact it has become a sex-organ or gonad, and "alternation of generations" has been transformed into "division of labour." Interesting divergences from the ordinary kinds of polymorph ism typical of the Hydroida and Siphonophora are found among the Trachylina, Scyphozoa, and in forms such as Millepora. In Millepora a polyp, be it a gastrozooid or a dactylozooid, becomes directly transformed under the influence of the immigrating sex cells into a medusa, instead of the medusa being formed from an independent bud. The polyp is a changeling. In the Trachylina the same process takes place, but in this case it is a pelagic or parasitic larval polyp which turns into a jellyfish, not an adult member of a colony. Among the Scyphozoa a unique condition exists; here the alternation of generations is very marked, and the medusa arises from the polyp direct ; but in this case the polyp divides itself transversely into a series of superimposed saucer like sections, which separate from it one by one, each becoming a medusa. A single polyp has therefore produced not one but several medusae. This is partly paralleled by a blastostyle, for if the latter be a polyp it produces by budding, several medusae; but the scyphozoan polyp achieves the same end quite differently.
The Anthozoa are the least interesting of the Coelenterata from the point of view of polymorphism. They possess no medu soid form and among their polyps little polymorphic variation in form occurs. In certain colonial forms ordinary polyps and siphonozooids co-exist, and there is also a distinction in some colonies between an original axial polyp and those subsequently formed. In the coral Fungia part of the life-cycle in some respects resembles the strobilization of the Scyphozoa.
Polymorphism is chiefly interesting as an example of the extraor dinary ability of animals to produce an almost infinite number of variations upon a given theme. It has also been much employed as a subject for argument, however, and it should be mentioned here that the viewpoint regarding it which has been adopted in all the articles on Coelenterata in this Encyclopædia is not univer sally accepted. The argument here adopted has been that a siphonophore or a compound Hydroid is a colony and that its parts represent, morphologically speaking, individuals connected by a common intermediate tissue, the coenosarc. This view may appear to be far-fetched when one contemplates a sporosac, which is in effect a gonad, since it involves the claim that this structure represents not an organ but an animal. It is also true that in some cases, such as those of meandrine corals, it is difficult to decide whether a polyp with a dozen mouths represents one polyp or twelve. But taking the whole story, as presented to us throughout the Coelenterate series, as a unity, the interpretation of the sys tems encountered as colonies of individuals appears to be the soundest working hypothesis. The alternative view, that every thing which results from the development of a single egg is an individual animal, however much it may subdivide asexually, and that gastrozooids, bracts, and the rest are nothing more than the organs of this animal, seems relatively satisfactory when applied to such a form as the adult Velella. But when one applies it to a free-swimming medusa it becomes reduced to an absurdity—is a jellyfish, an organism provided with mouth and stomach, canals, velum, tentacles and sense-organs, simply a moving organ and not an animal at all? Further, one may meet on many shores hosts of sea anemones, none of which have developed from eggs— each is an independent organism, not part of a colony, and each has been produced asexually by fission. Are these anemones then "organs"? To reduce the term "organ" to such a level is to remove its actual meaning. It is interesting to note that Moser combines the two theories by regarding the Coelenterata other than Siphonophora as colonies of individuals, but the Siphono phora themselves as relatively primitive organisms, individuals comprising organs and the fore-runners of true colonies.
In this connection it should however be noted that in the above paragraphs the term "individual" has been used in a purely morphological sense—i.e., the individuals of a Siphonophore rep resent morphological individuals which have suffered a loss of independence similar to that of the component cells of any multi cellular animal, which latter may be regarded as having been derived originally from a colony of individual single-celled ani mals (protozoa). In a physiological sense on the other hand the Siphonophore colony is a unity and may be regarded as an indi vidual system. The status of such a system in relation to the wide general question of individuality is further discussed in the article on that subject.