The movements of the Medusae are well described by Allman[301] in his account of _Cladonema radiatum_:—"It is impossible to grow tired of watching this beautiful medusa; sometimes while dashing through the water with vigorous diastole and systole, it will all at once attach its grapples to the side of the vessel, and become suddenly arrested in its career, and then after a period of repose, during which its branched tentacles are thrown back over its umbrella and extended into long filaments which float, like some microscopic sea-weed in the water, it will once more free itself from its moorings and start off with renewed energy." The Medusa of _Clavatella_, "in its movements and mode of life, presents a marked contrast to the medusiform zooid of other Hydrozoa. The latter is active and mercurial, dancing gaily through the water by means of the vigorous strokes of its crystalline swimming-bell. The former strides leisurely along, or, using the adhesive discs as hands, climbs amongst the branches of the weed. In the latter stage of its existence it becomes stationary, fixing itself by means of its suckers; and {268}thus it remains, the capitate arms standing out rigidly, like the rays of a starfish, until the embryos are ready to escape."[302] Among the Gymnoblastea there are many examples of a curious association of the Hydroid with some other living animals. Thus _Hydractinia_ is very often found on the shells carried by living Hermit crabs, _Dicoryne_ on the shells of various Molluscs, _Tubularia_ has been found on a Cephalopod, and _Ectopleura_ (a Corymorphid) on the carapace of a crab. There is but little evidence, however, that in these cases the association is anything more than accidental. The occurrence of the curious species, _Lar sabellarum_, on the tubes of _Sabella_, of _Campaniclava cleodorae_ on the living shells of the pelagic Mollusc _Cleodora cuspidata_, and of a _Gorgonia_ on the tubes of _Tubularia parasitica_, appear to be cases in which there is some mutual relationship between the two comrades. The genus _Stylactis_, however, affords some of the most interesting examples of mutualism. Thus _Stylactis vermicola_ is found only on the back of an _Aphrodite_ that lives at the great depth of 2900 fathoms. _S. spongicola_ and _S. abyssicola_ are found associated with certain deep-sea Horny Sponges. _S. minoi_ is spread over the skin of the little rock perch _Minous inermis_, which is found at depths of from 45 to 150 fathoms in the Indian seas.