74 



HYDROZOA 



Order 6. Siphoiiopftora. These are II ydromedusce in 

 which hydriform persons alone (Velelhi) 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 spurosacs and 

 only exceptionally attain full de- 

 velopment so as to be liberated 

 from the colony as free-swimming 

 medusa; (Velella, as Chrysvmitra; 

 Physalia, only liberating female 

 medusce). The medusiform persons, 

 where sufficiently developed, exhibit 

 the velum characteristic of Hydro- 

 mechtsie; 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 FlG 54 _ Portion of tl , conral . 

 bisenal or multiserial range of swim- 

 ming-bells (nectocalyces = medusee 

 with suppression of manubriuui, 

 tentacles, and sense-orgaus) are 

 placed. Covering pieces (hydro- 

 phyllia, reduced medusas) 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 Physalidtx the stem is con- 

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



In in Df Astylits subviri'.lit 

 (one of the Stiilasta /././ 1, 

 showing cyclosyst.unis placed 

 at intervals on the branches, 

 each with a central jj;astro- 

 pory and zone of slit-like dac- 

 tyloporcs. (After Moseley.) 



Ftfj. 55. Diagrams illustrating the .successive stages in the development of the 

 cyclnsysteins nf thr .S7v/".^c/-('Ar. 1, .S/""'" 1 '"/""'" <tic/it/it,i. L', ::, J//H/'NJV/ 

 tiobilis. 4, Allofxini in'vfinidft. 5, Alliyo/'d ini:ini>'i n. li, Astyliu tubviridis. 

 7, Ditlichopont COCCiliea. -s st\lr; <!/>. lUirtylupnre ; <//'. u r astr<ipol-e; 6,illfig.(J. 



inner horseshoe-shaped month of gastropore. (After Must-ley.) 



calyces and hydrophyllia are absent. In the sub-order 

 Cafycophoridce 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 Diphyida; and lead an independent life as 

 Eudoxice. 



In the suborder Discoidic the stem is converted into a 

 flattened disc with a system of caualicular 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 

 colonj, when they are known as Chrysomitra. 



FIG. 5f>. Diagram showing possible modifications of medusiform and hydri. 

 form persons of a colony of Siphonophoro. n, pneumatocyst; k, necto. 

 calyces (swimming bells); /, hydiupliyllium (covering-piece); /, generative, 

 medusiform person; p, dsctylozooid with attached tentacle, k ; e, nutritive 

 hydriform pcrsnn, with branched grappling tentacle, /; wt, stem. The thick 

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



The Sijilioiwpfwra alone, amongst the colonies formed by Hijdrozoa, 

 exhibit a high degree of division of labour and consequent individua- 

 tinn. 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 

 \\liii-h they attain (compare Cristatdla among Poli/zoa). VdeUa 

 and Pliiimiliu are occasionally seen on the southern and western 

 shores of England, but as a rule tin- SiphonopJiora are met with only 

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

 the Siphonopkora are assigned n distinct position among the Hydro- 

 zoa, side by side with the l[ijdromcdusa'-o.m\ Scyphomcdusce ; their 

 interpretation as floating colonies of Hijdromednsa', an interpre- 

 tation necessitated by the structure of their medusiform persons, 

 forbids their separation from that group. 



FOSSIL HYDKOZOA. Tlicresearuhesof Moseley have neces- 

 sitated a redistribution of the group of Aitthosoa known as 

 the Tubulate. Among these appear to be a few llydro- 

 coralliitti', which occur in the fossil state. The Paheozoic 

 forms known as graptolites are by some authors assigned 

 to the IfydroMt, 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 ScyjihomeJiisw have been obtained from the 

 Rolcnhofen slatus (Jurassic); excepting these, no noteworthy 

 extinct Hydruiua are known (see Haeckel in Zeitsch. iviss. 



