74 



HYDROZOA 



Order 6. SiphonopJiora. — These are Uydromedusoe in 

 whicli hydriform persons alone (Velella) or hydriforni 

 persons and sterile medusiform persons are united, under 

 many special modifications of form, to oonstitute floating 

 colonies of very definite shape and constitution. In 

 addition to these are developed medusiform sexual persons 

 which usually are spurosacs and 

 only exceptionally attain full de- 

 velopment so as to be liberated 

 from the colony as free-swimming 

 medusae {Velella, as Ghrysomitra; 

 Physalia, only liberating female 

 medusse). The medusiform persons, 

 wbere sufficiently developed, exhibit 

 the velum characteristic of Hydro- 

 med2isce; the larger mouth-bearing 

 nydriform persons, whicli 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, f). In the sub- 

 order FkysophoridcB (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 p,^, 64._portion of the coral- 

 biserial or multiserial range of swim- lum of Astyim subviruu 

 ming-bells (nectocalyces = medusae 

 with suppression of manubrium, 

 tentacles, and sense-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 Physalidae the stem is con- 

 verted into an air-sac, enormously enlarged, and the necto- 



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 Diiihyidw, and lead an independent life as 

 EudoxicB. 



In the suborder Discoidm 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 Ghrysomitra. 



(one of the Stylasteridce), 

 sliowing cyclosystems placed 

 at. intervals on the hranches, 

 ea«h with a central gastro- 

 pore and zone of slit-like dac- 

 tylopores. (After Moseley.) 



Fig S5 —Diagrams illustrating the successive stages in the development of the 

 cyclosystems of the Stylasteridce. 1, Sporadopora dichotoma. 2, 3, Allopora 

 nobilis 4 Allopora prof imda. 5, Allopora miniacea. 6, Astylus subviridis. 

 7 Disiichopora cocctnea. «, style; d^, dactylopore ; sp, gastropore ; 6, inflg. 6, 

 inner horseshoe-shaped mouth of gastropore. (After Moseley.) 



calyces and hydrophyllia are absent. In the sub-order 

 Calycophoridm the air-sac is not developed, the nectocalyces 

 are in a biserial group, or reduced to two or to one. 

 Dactylozooids are wanting. The modifi ed persons (append- 

 ages, Huxley) arise from the stem in groups, and can be 

 withdrawn into the cavity of a swimming-bell (fig. 57, B). 



Fig. 56.— Diagram showing possible modifications of medusiform and hydri: 

 form persons of a colony of Siphonophbra. n, pneumatocyst; *, necto<' 

 calyces (swimming bells); I, hydi ophylliura (covering-piece); f, generativq 

 medusiform person ; g, dactylozooid with attached tentacle, * ; e, nutritive 

 hydriform person, M-ith branched grappling tentacle, /;"m, stem. Thethicl^ 

 black line represents endoderm, the thinner line ectoderm. (After Allman.) 



The SiphonopJioraaXone, amongst the colonies formed by Hydrozoa, 

 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 defined individuality 

 which they attain (compart Gristatella among Polyzoa), Velella 

 and Physalia are occasionally seen on the southern and western 

 shores of England, but as a rule the Siphoncphoraaie met with only 

 in the open ocean and in the Mediterranean. By some authorities 

 the Siphonqphora are assigned a distinct position among the Bydro- 

 zoa, side by side with the Hydromedusas and ScyphomcdiiscB ; their 

 interpretation as floating colonies of Hydromedusce, an interpre- 

 tation necessitated by the structure of their medusiform persons, 

 forbids their separation from that group. 



Fossil Hydeozoa. — The researches of Moseley have neces- 

 sitated a redistribution of the group of Aiithozoa known as 

 the I'abulata. Among these appear to be a few Hydro- 

 coralliiKB, 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 Scypkomedusce have been obtained from the 

 Solenhofen slates (Jurassic); excepting these, no noteworthy 

 extinct Hydrozoa are known (see Haeckel in Zeitsch. wiss. 



