Hydrocaulus, and forms a cup, the hydrangium or hydrotheca (h, t), standing off from the body, into which the hydranth can be retracted for shelter and protection.
- 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 gymnoblastic polyp usually has a distinct perisarc investing the hydrorhiza and the hydrocaulus, sometimes also the hydranth as far as the bases of the tentacles (Bimeria); but in such cases the perisarc forms a closely-fitting investment or cuticule on the hydranth, never a hydrotheca standing off from it, as in the next sub-order.
An important variation is seen, in the form of the hydrotheca itself, which may come off from the main stem by a stalk, as in Obelia, or may be sessile, without a stalk, as in Sertularia.
I, The horny cup or hydrotheca of the hydriform persons; 1, medusiform person springing from m, a modified hydriform person (blastostyle); n, the horny case or gonangium enclosing the blastostyle and its buds.
This and the hydrotheca i give origin to the name Calyptoblastea.
- Diagrams to show the mode of formation of the Hydrotheca and Gonotheca in Calyptoblastic Hydroids.
Each such projection is regarded as representing a cup or hydrotheca, similar to those borne by a calyptoblastic hydroid, such as Sertularia.
A-D are stages common to both; from D arises the hydrotheca (E) or the gonotheca (F); th, theca; st, stomach; 1, tentacles; m, mouth; mb, medusa-buds.