The above comparison further indicates that the scyphistoma should not be regarded as a polyp but rather as a medusoid organism.
In the Leptolinae we must first substitute polyp for actinula, and then a condition is found which can be compared to the case of Cunina parasitica or Gonionemus, if we suppose that neither the parent-actinula (i.e.
The genus Myriothela is a solitary polyp with scattered capitate tentacles, producing sporosacs.
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.
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.
II, A) the polyp bears two tentacles only.
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.
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.
Whereas primitively any polyp in a colony may produce medusa-buds, in many hydroid colonies medusae are budded only by certain polyps termed blastostyles (fig.
The most familiar Anthozoan is the common sea-anemone, Actinia equina, L., and it will serve, although it does not form a skeleton or corallum, as a good example of the structure of a typical Anthozoan polyp or zooid.