Of the medusa differs only in greater elaboration and differentiation of the cell-elements, which are also more concentrated to form distinct tissues.
If the bud, however, is destined to give rise not to a free medusa, but to a gonophore, the development is similar but becomes arrested at various points, according to the degree to which the gonophore is degenerate.
The budding of this medusa has been worked out in detail by Chun (Hydrozoa, ), to whom the reader must be referred for the interesting laws of budding regulating the sequence and order of formation of the buds.
According to another story, her son Perseus, on his return with the head of Medusa, finding his mother persecuted by Polydectes, turned him into stone, and took Danae back with him to Argos.
As already stated, there occur in the Hydromedusae two distinct types of person, the polyp and the medusa; and either of them is capable of non-sexual reproduction by budding, a process which may lead to the formation of colonies, composed of more or fewer individuals combined and connected together.
- Lar sabellarum and two stages spread out in one plane, of its Medusa, Willia stellata.
The nervous system of the medusa consists of sub-epithelial ganglion-cells, which form, in the first place, a diffuse plexus of nervous tissue, as in the polyp, but developed chiefly on the subumbral surface; and which are concentrated, in the second place, to form a definite central nervous system, never found in the polyp. In Hydromedusae the central nervous system forms two concentric nerverings at the margin of the umbrella, near the base of the velum.
The possession of definite senseorgans at once distinguishes the medusa from the polyp, in which they are never found.
The endoderm of the medusa shows the same general types of structure as in the polyp, described above.
(In Hydra, on the other hand, the individual is usually hermaphrodite.) The medusa always reproduces itself sexually, and in some cases non-sexually also.
The direct method of medusa-budding only differs from the polyp-bud by its greater complexity of parts and organs.
- Diagrams of Medusa budding with the formation of an entocodon.
Finally, a mouth is formed by breaking through at the apex of the manubrium, and the now fully-formed medusa becomes separated by rupture of the stalk of the bud and swims away.
It is seen from the foregoing account of medusa - budding that the entocodon is a very important constituent of the bud, furnishing some of the most essential portions of the medusa; its cavity becomes the subumbral cavity, and its lining furnishes the ectodermal epithelium of the manubrium and of the sub-umbral cavity as far as the edge of the velum.
If the three principal organ-systems of the medusa, namely mouth, tentacles and umbrella, be considered in the light of phylogeny, it is evident that the manubrium bearing the mouth must be the oldest, as representing a common property of all the Coelentera, even of the gastrula embryo of all Enterozoa.
The special property of the medusa is the umbrella, distinguishing the medusa at once from other morphological types among the Coelentera.
The entocodon, however developed, gives rise at first to a closed cavity, representing a closing over of the umbrella, temporary in the bud destined to be a free medusa, but usually permanent in the sessile gonophore.
The possession of an entocodon proves the medusa-nature of the bud,.
2 may be produced on the medusa-buds before the latter are set free as medusae.
- Diagrams to show the significance of the Entocodon in Medusa-buds.
In Trachylinae the development produces always a medusa, and there is no polyp-stage.
The polyp is regarded, on this view, as a form phylogenetically older than the medusa, in short, as nothing more than a sessile actinula.
(2) With very few exceptions, the polyp is not the only type of individual that occurs, but alternates in the life-cycle of a given species, with a distinct type, the medusa, while in other cases the polyp-stage may be absent altogether, so that only medusa-individuals occur in the life-cycle.
(3) The gonads, whether formed in the polyp or the medusa, are developed in the ectoderm.
Hence both polyp and medusa present characters for classification, and a given species, genus or other taxonomic category may be defined by polyp-characters or medusa-characters or by both combined.
To establish the exact relationship it is necessary not only to breed but to rear the medusa, which cannot always be done in 1 In some cases hydroids have been reared in aquaria from ova of medusae, but these hydroids have not yet been found in the sea (Browne [Io a]).
The alternative is to fish all stages of the medusa in its growth in the open sea, a slow and laborious method in which the chance of error is very great, unless the series of stages is very complete.
The morphology of the group thus falls naturally into four sections - (I) the hydropolyp, (2) the polyp-colony, (3) the hydromedusa, (4) the medusa-colonies.
Since, however, medusa-colonies occur only in one group, the Siphonophora, and divergent views are held with regard to the morphological interpretation of the members of a siphonophore, only the first three of the above sub-divisions of hydromedusa morphology will be dealt with here in a general way, and the morphology of the Siphonophora will be considered under the heading of the group itself.
A, A hydriform person giving rise to medusiform person by budding from th margin of the disk; B, free swimming medusa (Steenstrupia of Forbes) detached from the same, with manubrial genitali.
On the other hand, the polyp .individuals produced by budding may remain permanently in connexion with the parent polyp, in which case sexual elements are never developed on polyp-individuals but only on medusa-individuals, and a true colony is formed.
After a time the polyps, or certain of them, produce by budding medusa-individuals, which sooner or later develop sexual elements; in some cases, however, the founder_ polyp remains solitary, that is to say, does not produce polypbuds, but only medusa-buds, from the first (Corymorpha, fig.
As a general rule polyp-buds are produced from the hydrorhiza and hydrocaulus, while medusa-buds are formed on the hydranth.
- Vacuolated Endoderm Cells of cartilaginous consistence from the axis of the tentacle of a Medusa (Cunina).
Hence it is convenient to consider the morphology of the medusa from these two aspects.
(a) The Medusa as an Independent Organism.
The remarkable medusa Mnestra parasites is ecto-para- '"' rp m sitic throughout life sr..
- A eginopsis hensenii, slightly magnified, showing the manner in which the tentacles are carried in life.) medusa usually has a characteristic method of carrying its tentacles.
The manubrium is absent altogether in the fresh-water medusa Limnocnida, in which the diameter of the mouth exceeds half that of the umbrella; on the other hand, the manubrium may attain a great length, owing to the centre of the sub-umbrella with the stomach being drawn into it, as it were, to form a long proboscis, as in Geryonia.
- Cladonema radiatum, the medusa walking on the basal branches of its tentacles (t), which are turned up over the body.
Mesogloea of the medusa is largely developed and of great thickness in the umbrella.
The second case gives a colony partly composed of polyp-individuals, partly of medusa-individuals, a possibility also realized in many colonies of Hydroidea.
From the bionomical point of view, the medusa is to be considered as a means of spreading the species, supplementing the deficiencies of the :" Ca sessile polyp. It may be, however, that increased reproductiveness becomes of greater importance to the species than wide diffu sion; such a condition FIG.
In this way the medusa sinks from an independent per sonality to an organ of the polyp-colony, becoming a so-called medusoid gonophore, or bearer of the reproductive organs, and losing gradually all organs necessary for an independent existence, namely those of sense, locomotion and nutrition.
That the sporosac should not be compared simply with the manubrium of the medusa, as is sometimes done.
The endodermal spadix (sp) of the sporosac represents the endoderm of the manubrium; the ectodermal lining of the sporosac (ex.) represents the ex-umbral ectoderm of the medusa; and the intervening layers, together with the sub-umbral cavity, have disappeared.
The medusa arises direct from the actinulastage and there is no entocodon formed, as in the budding described above.
In the majority of cases we do not know the polyp corresponding to a given medusa, or the medusa that arises from a given polyp.'
The third case gives a colony entirely composed of medusa-individuals, a possibility perhaps realized in the Siphonophora, which will be discussed in dealing with this group.