CLASSIFICATION OF HYDRACTINIA
Very common on Atlantic coast, Europe, U.S.A. (Alaska to San Francisco bay) and found in Sanjuan Island.
HABIT AND HABITAT
Hydractinia is a small, shallow-water, marine, colonial and sedentary hydroid, found on rocks, stones, fucus and piles.
GENERAL CHARACTERSTICS OF HYDRACTINIA
- It consists of vertical colonies arising from hydrorhiza.
- Polyps arise separately from incrusted spiny hydrorhiza.
- It is noteworthy that stolons or hydrorhizae are fused, forming brown encrustation enclosed in perisarc which is raised into several spines.
- Perisarc is confined only in hydrorhiza.
- It shows polymorphism.
- Animal contains several zooids borne on unbranched pedicles.
- Zooids are of four kinds (i) gastrozooids (feeding polyps) having single whorl of tentacles and mouth, (ii) gonozooids (reproductive polyps) bearing clusters of sporosacs containing eggs. (iii) dactylozooids (protective polyps) and (iv) skeletozooids.
H. echinata is found on gastropodan shells inhabited by hermit-crabs, which show commensalism but, according to Schijifsma (1935), the relationship is merely epizoic or facultative.
the specimen has vertical zooids and all above features hence it is Hydractinia.