A type of the Rhizophysina is the genus Rhizophysa. The Physalina comprise the families Physalidae and Epibulidae, of which the types are Physalia (figs. 74, 75) and Epibulia, respectively. Physalia, known commonly as the Portuguese man-of-war, is remarkable for its great size, its brilliant colours, and its terrible stinging powers.
Bibliography.—In addition to the works cited below, see the general works cited in the article Hydrozoa, in some of which very full bibliographies will be found.
1. G. J. Allman, “A Monograph of the Gymnoblastic or Tubularian Hydroids,” Ray Society (1871-1872); 2. A. Brauer, “Über die Entwickelung von Hydra,” Zeitschr. f. wiss. Zool. lii. (1891), pp. 169-216, pls. ix.-xii.; 3. “Über die Entstehung der Geschlechtsprodukte und die Entwickelung von Tubularia mesembryanthemum Allm.,” t.c. pp. 551-579, pls. xxxiii.-xxxv.; 4. W. K. Brooks, “The Life-History of the Hydromedusae: a discussion of the Origin of the Medusae, and of the significance of Metagenesis,” Mem. Boston Soc. Nat. Hist. iii. (1886), pp. 259-430, pis. xxxvii.-xliv.; 5. “The Sensory Clubs of Cordyli of Laodice,” Journ. Morphology, x. (1895), pp. 287-304, pl. xvii.; 6. E. T. Browne, “On British Hydroids and Medusae,” Proc. Zool. Soc. (1896), pp. 459-500, pls. xvi., xvii., (1897), pp. 816-835, pls. xlviii. xlix. 12 text-figs.; 7. “Biscayan Medusae,” Trans. Linn. Soc. x. (1906), pp. 163-187, pl. xiii.; 8. “Medusae” in Herdman, Rep. Pearl Oyster Fisheries, Gulf of Manaar, iv. (1905), pp. 131-166, 4 pls.; 9. “Hydromedusae with a Revision of the Williadae and Petasidae,” Fauna and Geogr. Maldive and Laccadive Archipelagos, ii. (1904), pp. 722-749, pls. liv.-lvii.; 10. “On the Freshwater Medusa liberated by Microhydra ryderi, Potts, and a Comparison with Limnocodium,” Quart. Journ. Micr. Sci. I (1906), pp. 635, 645, pl. xxxvii.; 10a. “On the Freshwater Medusa Limnocnida tanganicae” Budgett Memorial Volume (Cambridge, 1908, pp. 471-482, pl. xxviii.; 11. C. Claus, “Über die Struktur der Muskelzellen und über den Körperbau von Mnestra parasites Krohn,” Verhandl. zool. bot. Ges. Wien, xxv. (1876), pp. 9-12, pl. i.; 11a. C. Dawydov, “Hydroctena salenskii,” Mém. Acad. Imp. St. Pétersbourg (viii.) xiv. No. 9 (1903), 17 pp., 1 pl.; 12. A. Dendy, “On a Free-swimming Hydroid, Pelagohydra mirabilis,” n. gen. et sp., Quart. Journ. Micr. Sci. xlvi. (1903), pp. 1-24, pls. i. ii.; 13. H. Driesch, “Tektonische Studien an Hydroidpolypen,” (1) Jen. Zeitschr., xxiv. (1890), pp. 189-226, 12 figs.; (2) t.c. pp. 657-688, 6 figs.; (3) ibid. xxv. (1891), pp. 467-479, 3 figs.; 14. G. Duplessis, “On Campanularia volubilis,” Soc. Vaud. Bull. 13 (Lausanne, 1874-1875); 15. J. W. Fewkes, “On Mnestra,” Amer. Natural., xviii. (1884), pp. 197-198, 3 figs.; 16. S. Goto, “Dendrocoryne Inaba, Vertreterin einer neuen Familie der Hydromedusen,” Annot. Zool. Tokyo, i. (1897), pp. 93-104, pl. vi., figs. 106-113; 17. “The Craspe dote Medusa Olindias and some of its Natural Allies,” Mark Anniversary Volume (New York, 1903), pp. 1-22, 3 pls.; 18. H. Grenacher, “Über die Nesselkapseln von Hydra,” Zool. Anz. xviii. (1895), pp. 310-321, 7 figs.; 19. R. T. Günther, “On the Structure and Affinities of Mnestra parasites Krohn; with a revision of the Classification of the Cladonemidae,” Mitt. Stat. Neapel, xvi. (1903), pp. 35-62, pls. ii. iii.; 20. E. Haeckel, “Das System der Medusen,” Denkschr. med.-nat.-wiss. Ges. (Jena, 1879-1881); 21. “Deep Sea Medusae,” in Reports of the Challenger Expedition, Zool. iv. pt. 2 (London, 1882); 22. P. Hallez, “Bougainvillia fruticosa Allm. est le faciès d’eau agitée du Bougainvillia ramosa Van Ben.” C.-R. Acad. Sci. Paris, cxl. (1905), pp. 457-459; 23. O. & R. Hertwig, Der Organismus der Medusen (Jena, 1878), 70 pp., 3 pls.; 24. Das Nervensystem und die Sinnesorgane der Medusen (Leipzig, 1878), 186 pp., 10 pls.; 25. S. J. Hickson, “The Medusae of Millepora,” Proc. Roy. Soc. lxvi. (1899), pp. 6-10, 10 figs.; 26. T. Hincks, A History of British Hydroid Zoophytes (2 vols., London, 1868); 27. N. Iwanzov, “Über den Bau, die Wirkungsweise und die Entwickelung der Nesselkapseln von Coelenteraten,” Bull. Soc. Imp. Natural, Moscou (1896), pp. 323-355, 4 pls.; 28. C. F. Jickeli, “Der Bau der Hydroidpolypen,” (1) Morph. Jahrbuch, viii. (1883), pp. 373-416, pls. xvi.-xviii.; (2) t.c., pp. 580-680, pls. xxv.-xxviii.; 29. Albert Lang, “Über die Knospung bei Hydra und einigen Hydropolypen,” Zeitschr. f. wiss. Zool. liv. (1892), pp. 365-384, pl. xvii.; 30. Arnold Lang, “Gastroblasta Raffaelei. Eine durch eine Art unvollständiger Theilung entstehende Medusen-Kolonie,” Jena Zeitschr. xix. (1886), pp. 735-762, pls. xx., xxi.; 31. A. Linko, “Observations sur les méduses de la mer Blanche,” Trav. Soc. Imp. Nat. St Pétersbourg, xxix. (1899); 32. “Über den Bau der Augen bei den Hydromedusen,” Zapiski Imp. Akad. Nauk (Mém. Acad. Imp. Sci.) St Pétersbourg (8) x. 3 (1900), 23 pp., 2 pls.; 33. O. Maas, “Die craspedoten Medusen,” in Ergebn. Plankton Expedition, ii. (Kiel and Leipzig, 1893), 107 pp., 8 pls., 3 figs.; 34. “Die Medusen,” Mem. Mus. Comp. Zool. Harvard, xxiii. (1897), i.; 35. “On Hydroctena,” Zool. Centralbl. xi. (1904), pp. 240-243; 36. “Revision des méduses appartenant aux familles des Cunanthidae et des Aeginidae, et groupement nouveau des genres,” Bull. Mus. Monaco, v. (1904), 8 pp.; 37. “Revision der Cannotiden Haeckels,” SB. K. Bayer. Akad. xxxiv. (1904), pp. 421-445; 38. “Meduses,” Result. Camp. Sci. Monaco, xxviii. (1904), 71 pp., 6 pls.; 39. “Die craspedoten Medusen der Siboga-Expedition,” Uitkomst. Siboga-Exped. x. (1905), 84 pp., 14 pls.; 40. “Die arktischen Medusen (ausschliesslich der Polypomedusen),” Fauna arctica, iv. (1906), pp. 479-526; 41. C. Mereschkowsky, “On a new Genus of Hydroids (Monobrachium) from the White Sea, with a short description of other new Hydroids,” Ann. Mag. Nat. Hist. (4) xx. (1877), pp. 220-229, pls. v. vi.; 42. E. Metchinkoft, “Studien über die Entwickelung der Medusen und Siphonophoren,” Zeitschr. f. wiss. Zool. xxiv. (1874), pp. 15-83, pls. i.-xii.; 43. “Vergleichend-embryologische Studien” (Geryoniden, Cunina), ibid. xxxvi. (1882), pp. 433-458, pl. xxviii.; 44. Embryologische Studien an Medusen (Vienna, 1886), 150 pp., 12 pls., 10 figs.; 45. “Medusologische Mittheilungen,” Arb. zool. Inst. Wien, vi. (1886), pp. 237-266, pls. xxii. xxiii.; 46. L. Murbach, “Beiträge zur Kenntnis der Anatomie und Entwickelung der Nesselorgane der Hydroiden,” Arch. f. Naturgesch. lx. i. (1894), pp. 217-254, pl. xii.; 47. “Preliminary Note on the Life-History of Gonionemus,” Journ. Morph. xi. (1895), pp. 493-496; 48. L. Murbach and C. Shearer, “On Medusae from the Coast of British Columbia and Alaska,” Proc. Zool. Soc. (1903), ii. pp. 164-191, pls. xvii.-xxii.; 49. H. F. Perkins, “The Development of Gonionema murbachii,” Proc. Acad. Nat. Sci. Philadelphia (1902), pp. 750-790, pls. xxxi-xxxiv.; 50. F. Schaudinn, “Über Haleremita cumulans, n. g. n. sp., einen marinen Hydroidpolypen,” SB. Ges. natforsch. Freunde Berlin (1894), pp. 226-234, 8 figs.; 51. F. E. Schulze, “On the Structure and Arrangement of the Soft Parts in Euplectella aspergillum” (Amphibrachium), Tr. R. Soc. Edinburgh, xxix. (1880), pp. 661-673, pl. xvii.; 52. O. Seeliger, “Über das Verhalten der Keimblätter bei der Knospung der Cölenteraten,” Zeitschr. f. wiss. Zool. lviii. (1894), pp. 152-188, pls. vii.-ix.; 53. W. B. Spencer, “A new Family of Hydroidea (Clathrozoon), together with a description of the Structure of a new Species of Plumularia,” Trans. Roy. Soc. Victoria (1890), pp. 121-140, 7 pls.; 54. M. Ussow, “A new Form of Fresh-water Coelenterate” (Polypodium), Ann. Mag. Nat. Hist. (5) xviii. (1886), pp. 110-124, pl. iv.; 55. E. Vanhöffen, “Versuch einer natürlichen Gruppierung der Anthomedusen,” Zool. Anzeiger, xiv. (1891), pp. 439-446; 56. C. Viguier, “Études sur les animaux inférieurs de la baie d’Alger” (Tetraplatia), Arch. Zool. Exp. Gen. viii. (1890), pp. 101-142, pls. vii.-ix.; 57. J. Wagner, “Recherches sur l’organisation de Monobrachium parasiticum Méréjk,” Arch. biol. x. (1890), pp. 273-309, pls. viii. ix.; 58. A. Weismann, Die Entstehung der Sexualzellen bei den Hydromedusen (Jena, 1883); 59. R. Woltereck, “Beiträge zur Ontogenie und Ableitung des Siphonophorenstocks,” Zeitschr. f. wiss. Zool. lxxxii. (1905), pp. 611-637, 21 text-figs.; 60. J. Wulfert, “Die Embryonalentwickelung von Gonothyraea loveni Allm.,” Zeitschr. f. wiss. Zool. lxxi. (1902), pp. 296-326, pls. xvi.-xviii.
(E. A. M.)
[1] In some cases hydroids have been reared in aquaria from ova of medusae, but these hydroids have not yet been found in the sea (Browne [10 a]).
[2] The numbers in square brackets [] refer to the bibliography at the end of this article; but when the number is preceded by the word Hydrozoa, it refers to the bibliography at the end of the article [Hydrozoa].
HYDROMETER (Gr. ὕδωρ, water, and μέτρον, a measure), an instrument for determining the density of bodies, generally of fluids, but in some cases of solids. When a body floats in a fluid under the action of gravity, the weight of the body is equal to that of the fluid which it displaces (see [Hydromechanics]). It is upon this principle that the hydrometer is constructed, and it obviously admits of two modes of application in the case of fluids: either we may compare the weights of floating bodies which are capable of displacing the same volume of different fluids, or we may compare the volumes of the different fluids which are displaced by the same weight. In the latter case, the densities of the fluids will be inversely proportional to the volumes thus displaced.
The hydrometer is said by Synesius Cyreneus in his fifth letter to have been invented by Hypatia at Alexandria,[1] but appears to have been neglected until it was reinvented by Robert Boyle, whose “New Essay Instrument,” as described in the Phil. Trans. for June 1675, differs in no essential particular from Nicholson’s hydrometer. This instrument was devised for the purpose of detecting counterfeit coin, especially guineas and half-guineas. In the first section of the paper (Phil. Trans. No. 115, p. 329) the author refers to a glass instrument exhibited by himself many years before, and “consisting of a bubble furnished with a long and slender stem, which was to be put into several liquors, to compare and estimate their specific gravities.” This seems to be the first reference to the hydrometer in modern times.