The perisarc when, present invests the hydrorhiza and hydrocaulus; it may stop short below the hydranth, or it may extend farther.
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.
As a general rule polyp-buds are produced from the hydrorhiza and hydrocaulus, while medusa-buds are formed on the hydranth.
Common flesh, which cannot be assigned more to one individual than another, but consists of a more or less complicated network of tubes, corresponding to the hydrocaulus and hydrorhiza of the primitive independent polypindividual.