The foot by which it is attached often sends out root-like processes - the hydrorhiza (c).
The perisarc when, present invests the hydrorhiza and hydrocaulus; it may stop short below the hydranth, or it may extend farther.
- 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.
In the first place, buds may be produced only from the hydrorhiza, which grows out and branches to form a basal stolon, typically net-like, spreading over the substratum to which the founderpolyp attached itself.
In massive colonies of this kind no sharp distinction can be drawn between hydrorhiza and hydro-, .
Budding from the hydrocaulus may be combined with budding from the hydrorhiza, so that numer ous branching colonies arise from a common basal stolon.
The planula may fix itself (I) by one end, and then becomes the hydrocaulus and hydranth, while the hydrorhiza grows out from the base; or (2) partly by one side and then gives rise to Modified from a plate by L.
The hydrorhiza as well as to the other Agassiz, Contributions to Nat.
In some hydroids the founder-polyp, developed from a planula after fixation, throws out numerous outgrowths from the base to form the hydrorhiza; these outgrowths may be radially arranged so as to form by contact or coalescence a flat plate.
It would be necessary to regard this structure as a secondary extension of the endoderm in the tentacle-web, on Allman's theory, or between the outgrowths of the hydrorhiza, on Mechnikov's hypothesis.
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.
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.
The hydroid genus Lafoea is remarkable for producing gonothecae on the hydrorhiza, each containing a blastostyle which bears a single gonophore; this portion of the colony was formerly regarded as an independent parasitic hydroid, and was named Coppinia.
- This family contains the single Australian species Clathrozoon wilsoni Spencer, in which a massive hydrorhiza Stage 4, seen in Plumularidae.
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.