(I) The polyp, when present, is without the strongly developed longitudinal retractor muscles, forming ridges (taeniolae) projecting into the digestive cavity, seen in the scyphistoma or scyphopolyp. (2) The medusa, when' present, has a velum and is hence said to be craspedote; the nervous system forms two continuous rings running above and below the velum; the margin of the umbrella is not lobed (except in Narcomedusae) but entire; there are characteristic differences in the sense-organs (see below, and Scyphomedusae); and gastral filaments (phacellae), subgenital pits, &c., are absent.
The development from the egg may be direct, or may take place with an alternation of generations (metagenesis), in which a non-sexual individual, the so-called scyphistoma or scyphopolyp, produces by budding the sexual medusae.
The larva after fixation changes into a polyp-like organism termed a scyphistoma or scyphopolyp (fig.
The internal gastric cavity of the scyphistoma is not a simple space as in the hydropolyp, but is subdivided by four ridges or taeniolae, arising one in each interradius (fig.
Each taeniola bears a strongly developed longitudinal muscle-band, stated by Claus and Chun to be developed from the endoderm, like the retractor muscles of the anthopolyp, but by other investigators it is affirmed that each retractor muscle of the scyphistoma arises from the lining of a funnel-shaped ectodermal ingrowth (" Septaltrichter ") growing down from the peristome inside each taeniola, in a manner similar to the infundibular cavities of Lucernaria, which in their turn are homologous with the sub f genital cavities of Scypho l A .` medusae.
B; lam ??`0` asserted, however, time, is produced in this way (monodisk strobilization); a circular furrow cuts off the upper, tentacle-bearing portion from the lower half of the scyphistoma (fig.
In this state the scyphistoma is termed a strobila.
The medusae produced by strobilization of the scyphistoma are of a peculiar type termed Ephyrae (fig.
As preparations Scyphistoma of Chrysaora, with four perradial tentacles and horny basal perisarc.
Oral surface of later stage of scyphistoma of Aurelia, with commencement of four interradial tentacles.
Oral surface of a sixteententacled scyphistoma of Aurelia.
D, First constriction of the Aurelia scyphistoma to form the pile of ephyrae or young medusae.
The single ephyra carries the sixteen scyphistoma tentacles, which will atrophy and disappear.
For their formation the margin of the peristome of the scyphistoma grows out into eight lobes, four perradial, four interradial.
The sixteen tentacles of the scyphistoma disappear, and in the place of the four perradial and four interradial tentacles, the eight tentaculocysts of the adult are formed as outgrowths of the subumbral margin, independently of the tentacles of the scyphistoma (Friedemann).
(After Claus.) by Friedemann (3), a recent investigator of the subject, that the infundibular cavities appear late in the scyphistoma and have no relation either to the septal muscles or to the subgenital cavities of the adult.
The muscle-bands are very contractile, rendering the scyphistoma one of the most difficult of all organisms to preserve in an expanded condition.
As the scyphistoma grows the tentacles increase in number, four interradial and eight adradial being formed in addition to the four primary perradial tentacles (fig.
The scyphistoma of Nausithoe forms a branching network which grows in the sponge Esperella and forms the colonial polypoid organism named by Schulze Spongicola fistularis, by Allman Stephanoscyphus mirabilis.
Sooner or later, however, the scyphistoma produces free medusae by a process of transverse fission termed strobilization.
Thus in Pelagia the scyphistoma-stage is free-swimming and changes directly into the ephyra, which in its turn grows into the adult form.
On the other hand, such a form as Lucernaria or Haliclystus may be regarded simply as a scyphistoma which has become adult and mature.
The comparison of the metagenetic type of development, such as that of Aurelia, with the more primitive genera of Scyphomedusae, indicates clearly that the scyphistoma and ephyra are recapitulative larval stages which are represented by the adult forms of primitive genera, making such allowances as are necessary when comparing adult and larval forms. The metagenesis has arisen through the scyphistoma-larva acquiring the power of larval proliferation by budding.
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.
Above to left, young scyphistoma with four perradial tentacles.
Below to left, scyphistoma with sixteen tentacles and first constriction.
To the right, strobila condition of the scyphistoma, consisting of thirteen metameric segments; the uppermost still possesses the sixteen tentacles of the scyphistoma; the remainder have no tentacles, but are ephyrae, each with eight bifid arms (processes of the disc).