74 



HYDROZOA 



Order 6. Siphonophora. These are Hydromedusee 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 spurosacs and 

 only exceptionally attain full de- 

 velopment so as to be liberated 

 from the colony as free-swimming 

 medusaj (Velella, as Chrysomitra; 

 Physalia, only liberating female 

 medusas). The medusiform persons, 

 where sufficiently developed, exhibit 

 the velum characteristic of Hydro- 

 mediisce; the larger mouth-bearing 

 nydriform persons, which are some- 

 times the only representatives of 

 their kind, care remarkable for 

 differentiation into four regions, 

 a proboscis, a stomach, a basal ring, 

 and a short stalk on which the 

 tingle tentacle of great length is 

 situated (fig. 56, f). 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 Flo " 4 _ Portlon of thecoral . 

 biserial or multiserial range of swim- ium of Astyius 

 ming-bells (nectocalyces = medusae 

 with suppression of manubrium, 

 tentacles, and sense-organs) 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 suVorder Physalidte the stem is con- 

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



(onc of the StylasleriJa?), 

 showing cyclosystems placed 

 at intervals on the branches, 

 each with a central gastro- 

 pdre and zone of slit-like dac- 

 tylopores. (After Moseley.) 



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

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

 nobitis.-.4,Alloporaprofunda, 5, Allopora miniacea. G, Astyius subviridis. 

 7, Distichopora coccinea. s, style ; dp, dactylopore ; ffp, gastropore ; 6, in fig. 6. 

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



calyces and hydrophyllia are absent. In the sub-order 

 Calycoplwridce 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 

 Eudoxicc. 



In the suborder Discoidce the stem is converted into a 

 flattened disi; 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; 1-, necto. 

 calyces (swimming bells); /. hydiophyllium (covering-piece); f, generative, 

 medusifonn person ; y, 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 Allman.) 



The Siphonophora alone, 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 lias 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 (compare Cristatdla among Polyzoa). Velella 

 and Physalia are occasionally seen on the southern and western 

 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 SiphonopJtora are assigned a distinct position among the Hydro- 

 zoa, side by side with the Hydromcdusce nnd Scyphomcdusce ; their 

 interpretation as floating colonies of Hydromcdusa:, 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- 

 coralliitie, 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 Scyphomediisui have been obtained from the 

 Soleuhofen slates (Jurassic); excepting these, no noteworthy 

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



