5G4 HYDROZOA Order 6. Siphonophora. These are Ilydromedusce in which hydriform persons alone (Velella) or hydriform persons and sterile medusiform persons are united, under many special modifications of form, to constitute floating colonies of very definite shape and constitution. In addition to these are developed medusiform sexual persons which usually are sporosacs and only exceptionally attain full de velopment so as to be liberated from the colony as free-swimming medusae (Velella, as Chrysomitra] Physalia, only liberating femalo medusae). The medusiform persons, where sufficiently developed, exhibit the velum characteristic of Ilydro medusce; the larger mouth-bearing nydriform persons, which are some times the only representatives of their kind, are remarkable for differentiation into four regions, a proboscis, a stomach, a basal ring, and a short stalk on which the single tentacle of great length is situated (fig. 56, /). In the sub order Physophoridce (fig. 57, C) the persons are united by a short or long and spiral stem, terminated at one end by a flask-like air-sac (pneumatocyst); below the air-sac a r;Gi " 54 _ Poi . tion of thccoral . biserial or multiserial range of swim- ium of Astyius ming-bells (nectocalyces = medusce with suppression of manubrium, tentacles, and senss-organs) are placed. Covering pieces (hydro- phyllia, reduced medusae) and dactylozooids are affixed to the succeeding region of the stem, and alternate in definite order with the mouth-bearing hydriform persons (polyps or nutritive persons) and generative medusiform persons. In the sub-order Physalidoe the stem is con verted into an air-sac, enormously enlarged, and the necto- (one of the Styltutertdce), showing cyclosystems placed at intervals on the branches, each with a central gastro- pore and zone of slit-like dac- tylopores. (After Moseley.) FIG. 55. Diagrams illustrating the successive stages in the development of the cyclosystems of the Sty aiteridai. 1, Sporadopora dicholoma. 2, 3, AUopora nobilis. 4,Alloporaprofund<.i. 5, AUopora miniacea. 6, Astyius subviridis. 7, Dislichopora coccinea. t, style ; dp, dactylopore ; gp, gastropore; 6, in fig. 6, inner horseshoe-shaped mouth of gastropore. (After Moseley.) , calyces and hydrophyllia are absent. In the sub-order CalycophoridiB the air-sac is not developed, the nectocalyces are in a biserial group, or reduced to two or to one. Dactylozooids are wanting. The modified persons (append ages, Huxley) arise from the stem in groups, and can be withdrawn into the cavity of a swimming-bell (fig. 57, B). Each group consists of a nutritive person, with long ten tacle, of generative medusoids, and usually also an umbrella- shaped or funnel-like covering piece. The latter separate in some Diphyidce, and lead an independent life as Eudoxice. In the suborder Discoidce the stem is converted into a flattened disc with a system of canalicular cavities. Above this lies the air sac, a flattened reservoir of cartilaginous consistence. The hydriform persons depend from the disc, centrally a large nutritive person surrounded by smaller similar persons carrying at their bases the generative medusoids ; near the edge of the disc are dactylozooids. The medusoids develop into complete medusiform persons, and develop the genital products after liberation from the colony, when they are known as Chrysomitra. FIG. 56. Diagram showing possible modifications of medusiform and hydri form persons of a colony of Siphonophora. n, pneumatocyst; k, necto calyces (swimming bells); L hydrophyllium (covering-piece;; i, generative medusiform person ; g, dactylozooid with attached tentacle, h ; e, nutritive hydriform person, with branched grappling tentacle, /; m, stem. The thick black line represents endoderm, the thinner line ectoderm. (After Allrnan.) The Siphono2)Jtoraa}oTe, amongst the colonies formed by Hydro~oa, exhibit a high degree of division of labour and consequent individua- tion. The mode of origin of such colonies has been discussed above. The locomotive habit, as contrasted with the sessile habit of other colonies, is no doubt correlated with the sharply denned individuality which they attain (compare Cristatella among Polyzoa). Velella and Physalia are occasionally seen on the southern and vestern shores of England, but as a rule the Siphonophora are met with only in the open ocean and in the Mediterranean. By some authorities the Siphonophora are assigned a distinct position among the Hydro- zoa, side by side with the Hydromcdusce and Scyphomcdusce ; their interpretation as floating colonies of Hydromcdusce, an interpre tation necessitated by the structure of their medusiform persons, forbids their separation from that group. FOSSIL HYDROZOA. The researches of Moseley have neces sitated a redistribution of the group of Anthozoa known as the Tabulata. Among these appear to be a few Hydro- corallina 1 , which occur in the fossil state. The Palaeozoic forms known as graptolites are by some authors assigned to the Hydrozoa, but the grounds for placing them in this position are very slight, owing to the imperfect nature of the remains. A discussion of the small amount of structure which they present would be out of place here. Remarkable Scyphomedusce have been obtained from the Solenhofen slates (Jurassic); excepting these, no noteworthy
extinct Hydrozoa are known (see Haeckel in Zeitsch. wiss.
