HYDEACHNEA. 



[ 400 ] 



HYDROCYTIUM. 



are found attached to aquatic insects (fig. 

 29 y), as Nepa,Dytiscus, &c. 



H. geographica. Body spherical, black, 

 with spots and yellow points ; palpi red, 

 acute ; legs shorter than the body, black, 

 but red at the ends. 



H. concharum. Inhabits the pallial cavity 

 of the Naiades. 



BIBL. Duges, An. Sc. Nat. 2 se*r. i. ; Ger- 

 vais, Walckenaer 1 s Arachn. iii. ; Koch, 

 Deutschl. Crustac. ; Murray, EC. Ent. 157. 



HYDRACH'NEA (Water-spiders). A 

 family of Acarina. 



Palpi with the last joint unguiculate or 

 spinous ; two or four distinct ocelli ; coxae 

 broad, legs generally ciliated, natatory, the 

 posterior longest, tarsi with two claws. 



The characters of the genera must be 

 sought under the individual heads. See 

 ARRENURUS, ATAX, DIPLODONTUS, EYLAIS, 

 HYDRACHNA, and LIMNOCHARES. 



HYDRACTINTA, V. Bened. A genus 

 of Hydroid Polypi, of the family Hydrac- 

 tiniidae. 



Char. Polypidom incrusting ; polypes 

 claviform, tentacles in a single whorl at the 

 base of a conical proboscis. 



H. echinata. Polypidom rough with ser- 

 rated spines, whitish fleecy. On univalve 

 shells tenanted by the hermit crab. 



BIBL. Hincks, Brit. Zooph. p. 19. 



HYDRALLMAN'IA, Hincks. A genus 

 of Hydroid Zoophytes, f am. Sertulariidae. 



H. falcataPlumulariafalc. Johnst. 



BIBL. Hincks, Brit. Zooph. p. 273. 



HYDRAN'THEA, Hincks A genus of 

 Hydroid Polypi, fam. Atractylidae. 



H. margaritacea, white ; on Flustra 

 foliacea. 



BIBL. Hincks, Ann. N. H. 1863, xi. 45 j 

 and Br. Zooph. 99. 



HYDRIA'NUM, Rab. A genus of Pal- 

 mellaceous Algae. 



Char. Cells resembling those of Chara- 

 cium, but open at the ends ; endochrome 

 contracted, ultimately becoming resolved 

 into 2-4-8 zoogonidia. H. ovale (PI. 62. 

 fig. 5). 



12 species : freshwater ; adherent to other 

 Algae. 



BIBL. Rabenhorst, Fl. Alq. iii. p. 87. 



H Y'DRIAS, Ehr. A genus of Rotatoria, 

 of the family Philodinaea. 



Char. Eyes absent; neither proboscis 

 present, nor horn-like processes on the foot ; 

 rotatory organs two, placed at the ends of 

 two anterior processes of the body. 



H. cornigera (PI. 43. fig. 39). Body ovate, 



hyaline ; foot narrowed into the form of a 

 slightly forked tail ; freshwater ; length 

 1-190". 



Probably a young and imperfectly exa- 

 mined Philodina. Found in Egypt. 



BIBL. Ehrenberg, Infus. p. 483. 



HY'DRID^E. A family of Zoophytes, 

 order Hydroida. 



It contains the single genus Hydra. 



HYDROCHARIDA'CE^E. A familyof 

 Monocotyledonous Flowering Plants grow- 

 ing in water, interesting to the microscopist, 

 as affording very favourable opportunities of 

 viewing the circulation or rotation of the 

 cell-contents. The leaves of Vallisneria 

 spiralis, an Italian plant, which is readily 

 grown in jars of water indoors, are very fre- 

 quently used for this purpose ; the leaves 

 and sepals ofAnacharisAlsinastnim, a North 

 American plant, now naturalized in streams 

 in many parts of Britain, also show the cir- 

 culation well. The extremities of the roots 

 of Hydrocharis morsits-rance, a plant com- 

 mon, floating on the surface, in broad per- 

 manent ditches, are likewise adapted for the 

 purpose. The circulation consists of the 

 flowing movement of a layer of colourless 

 protoplasm over the inner surface of the 

 walls of the cells. Where, as in the leaves 

 of Vallisneria and Anacharis, the cells con- 

 tain green globules of chlorophyll, these 

 mostly adhere to the circulating mass, and 

 are carried round with it. The phenomenon 

 may be observed in uninjured young leaves 

 simply immersed in water by focusing care- 

 fully ; but in Vallisneria it is seen more 

 clearly in slices taken carefully parallel to 

 the surface of the leaf. The circulation 

 lasts a long time in these separate frag- 

 ments if they are kept wet. Sometimes it 

 is arrested by the preparation j in such cases 

 the application of a gentle heat often causes 

 it to recommence. It may be observed with 

 a power of 200 diameters ; but a higher is 

 requisite for minute investigation. (See 

 ROTATION.) 



HYDROCHORE CITES, Koch, = Hyd- 

 rachna, pt. 



HYDROCO'LEUM, Kiitz. A genus of 

 Oscillatoriaceee (Confervoid Algae), corre- 

 sponding to aquatic species of * Chthono- 

 blastus. H. helveticum (PI. 52. fig. 6). 



11 European species. 



BIBL. Kiitzing, Phyc. Gen. 196 ; Raben- 

 horst, Fl Alg. ii. p. 149. 



HYDROCY'TIUM, Al. Braun. A genus 

 of unicellular Algse, separated from that 

 author's Characium on account of the whole 



