(I) The polyp (hydropolyp) is of simple structure, typically much longer than broad, without ectodermal oesophagus or mesenteries, such as are seen in the anthopolyp (see article Anthozoa); the mouth is usually raised above the peristome on a short conical elevation or hypostome; the ectoderm is without cilia.
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.
The Hydropolyp (fig.
The architecture of the hydropolyp, simple though it be, furnishes a long series of variations affecting each part of the body.
- Diagram of a typical Hydropolyp. Hydranth; Hydrocaulus; Hydrorhiza; Tentacle; Perisarc, forming in the region ' of the hydranth a cup or hydrotheca(h, t), - which, however,is only found in polyps of the order Calyptoblastea.
The hypostome of the hydropolyp may be small, or, on the other hand, as in Eudendrium (Allman, loc. cit.
- The ectoderm degenerate medusiform persons or of the hydropolyp is chiefly sporosacs b.
In the hydropolyp the ectodermal muscle-fibres are always directed longitudinally.
The mesogloea in the hydropolyp is a thin elastic layer, in which may be lodged the muscular fibres and ganglion cells mentioned above, but which never contains any connective tissue or skeletogenous cells or any other kind of special mesogloeal corpuscles.
In the hydropolyp the body is typically elongated, the height of the column being far greater than the diameter.
The internal structural differences are even more characteristic. In the hydropolyp the blastopore of the embryo forms the adult mouth situated at the extremity of the hypostome, and the ectoderm and FIG.
Further, in the hydropolyp the digestive cavity either remains simple and undivided and circular in transverse section, or may show ridges projecting internally, which in this case are formed of endoderm alone, without any participation of the mesogloea.
In short, the hydropolyp is characterized by a more simple type of organization than the anthopolyp, and is in most respects less modified from the actinula type of structure.
The polyp (hydropolyp) is of simple structure, and never has an ectodermal oesophagus or mesenteries.
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.