HYDRODICTYON. 



[ 402 ] 



HYDROIDA. 



sent a slight tremulous oscillatoiy niove- 

 mt'ut. The cuticle of the parent cell then 

 cracks, allowing the inner softened layers 

 to swell out; the gonidia commence an 

 active trembling and jerking motion, not, 

 however, moving far from one spot ; after a 

 time they again come to rest, and become 

 united at certain points of their circum- 

 ference ; the green granules become fused 

 into a homogeneous mass, and the rudi- 

 ment of the first starch-granule soon ap- 

 pears, while the gonidia grow out into a 

 tubular form, acquire a cellulose membrane, 

 and collectively form a new net, which be- 

 comes free by the total solution of the pa- 

 rent cell. Tliese gonidia appear to possess 

 four short cilia ; their motion lasts about 

 half an hour ; from 7000 to_ 20,000 occur in 

 one cell : and thej' are distinguished by 

 Braun as macro(/on{clia. Other gonidia of 

 smaller size and longer shape, which he 

 calls microyrmidia, are furnished with four 

 long cilia and a red parietal spot. These 

 have a different history. From 30,000 to 

 100,000 appear in the parent cell, their de- 

 velopment presenting the same characters 

 as that of the macrof/onidia up to the time 

 when the motion begins. Then the micro- 

 (jonidia, unlike the net-forming VKtcror/oni- 

 dia, leave their parietal positions with a 

 whirling motion, and move through the 

 entire cavity of the parent cell, until at 

 length the membrane of the latter bulges 

 out in one or more places and bursts, and 

 the microgonidia leave the cavity in a 

 swarm. According to Colin, they are at 

 first enclosed in a thin mucilaginous pellicle 

 protruded before them, like the swarming 

 spores of Pediastrum. However, they 

 escape, become free, and swim about for a 

 long time, and are sujiposed to conjugate. 

 At length thej' come to rest, acquire an 

 outer coat, sink to the bottom, and remain 

 there heaped in green masses, like cells of 

 Protococcus, for a long period, forming 

 resting-spores. 



The rapidity of the growth of the JTi/dro- 

 didyon-nei by the above process is wonder- 

 ful ; the component cells of the net increase, 

 under favourable circumstances, to 600 times 

 tlieir original length in a few weeks. In 

 cultivated specimens, the whole history, 

 from the origin of a net to the production 

 of a new one, passes over in tliree or four 

 weel\s. The original size of the cells is 

 about 1-2500"; in the fully developed con- 

 dition they are about 1 to 4" long. 



PiBL. Vauchcr, Confcrves, ^2, pi. 9 ; 



Areschoug, Linrupa, xvi. 127, pi. 5 (1842) ; 

 Ilassall, AhfcB, 225, pi. 58 ; Braun, Ver/ihiy. 

 (Hay Soc. '185.3) ; A/y. UniceU. Sp. ' Nov. 

 1855, 55 ; Cohn, Nova Acta, xxiii. 207, pi. 

 19; Pringsheim, ^«-/. i?er. 1800; Qu. M. 

 Jn. 1802, 54, 104. 



HYDROGAS'TEUM, Desv. = Botby- 

 DirrM. 



HYDROI'DA.— The order of Hydroid 

 Polypes (Zoophytes). 



The members of this Order usually con- 

 sist of aggregate hydriform individuals 

 contained in cells or cups, springing from a 

 filiform branched polypidom ; propagation 

 gemmiparous and oviparous. Many new 

 names have been used for the component 

 structures, which may well be mentioned 

 here ; but we prefer the old ones. One of 

 the forms of non-sexual reproduction is the 

 zooid. Zooids ditler from organs in that 

 the zooid is an individual organism, which 

 may or may not be capable of individual 

 existence. A community of zooids in imion 

 with one another constitutes the hydrosnme. 

 Zooids are of two kinds : in one, destined 

 for the nutrition of the community, the as- 

 semblage is called the trophosome;t\\e oihex 

 gives origin to the generative elements — 

 ova and spermatozoa ; and the entire asso- 

 ciation of these generative zooids is the 

 yonosome. The trophosome is composed of 

 the hydranih and the liydruphyton. The 

 hydranth or polypite contains the digestive 

 sac ; the hydrophyton or ccenosarc is the 

 common basis -by which the general com- 

 munity is kept together. The hydrorhha 

 is the adherent base. The ultimate zooid, 

 which generates either the ova or the 

 spermatozoa, is the yonojjhore. The spo?-o- 

 sac is the gonophore without the umbrella. 

 The yonozoid is the sexual zooid, whether 

 fixed or detached and fitted for locomotive 

 life ; this is also known as a medusiform 

 gonophore, or planoblast. The ytmaxyium, 

 or (/onothcca, is an external receptacle in 

 which the gonophore is formed. (Pascoe.) 

 See Zoophytes. 



Ilincks divides the Order into 3 sub- 

 orders : — 



1. Athccata (Tubularina, Johnst.), in 

 which true thecse are absent. Fam. : 



Clavidas. 



Jlydractiuiidfe. 



I'odocorynidje. 



Ijavida^.. 



Coi'vui'l-e. 



Stauridiida). 



Clavatellida). 



jMyriothelidas. 



Eudeiidiiidie. 



Atiactylidie. 



TubulariidiT}. 



Penuariidaj. 



