Polyp 7 has proof sense, 1 o c oduced as its first bud, 8; as its second bud, a7, motion and nutriwhich starts a uniserial pinnule; and as a third t i on, until its bud I', which starts a biserial branch (I I'-VI') medusoid nature that repeats the structure of the main stem and and organization gives off pinnules.
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.
Here the medusoid, attached by the centre of its ex-umbral surface, has lost its velum and sub-umbral muscles, its sense organs and mouth, though still retaining rudimentary tentacles.
It has been maintained that the gonads of Hydra represent sporosacs or gonophores greatly reduced, with the last traces of medusoid structure completely obliterated.
There is, however, no evidence whatever for this, the gonads of Hydra being purely ectodermal structures, while all medusoid gono phores have an endodermal portion.
The entocodon is usually formed, proving the medusoid nature of the bud, but in sporosacs the entocodon may be rudimentary or absent altogether.
- Simple polyps which become sexually mature and which also reproduce non-sexually, but without any medusoid stage in the life-cycle.
The most usual condition, however, is that in which sessile medusoid gonophores or sporosacs are produced.
M, Medusoid gonophores.
Haeckel, on the other hand, is in accordance with Balfour in regarding a Siphonophore as a medusome, that is to say, as a colony composed of medusoid persons or organs entirely.
They typically present two structural forms, the non-sexual hydroid and the sexual medusoid; in such a case there is an alternation of generations (metagenesis), the hydroid giving rise to the medusoid by a sexual gemmation, the medusoid bearing sexual cells which develop into a hydroid.
In some other cases medusoid develops directly from medusoid (hypogenesis), whether by sexual cells or by gemmation.
In other cases the medusoid is hypogenetic, medusoid producing medusoid.
The sexual cells of the medusoid lie in the endoderm on interradii, that is, on the second set of radii accentuated in the course of development.
The Anthozoa differ from the Scyphomedusae in having no medusoid form; they all more or less resemble a sea-anemone, and may be termed actinioid.
The above comparison further indicates that the scyphistoma should not be regarded as a polyp but rather as a medusoid organism.
Hence the absence of sense-organs in the scyphistoma does not necessarily disprove its medusoid character, while its anatomical structure resembles that of a simple scyphomedusa, such as Lucernaria, rather than that of a polyp.
Such an animal, if it ever existed, probably lived near the surface of the sea, and even here it may have changed its medusoid mode of locomotion for one in the direction of its mouth.