HYDRACHNA. 



[ 330 ] 



HYDRODICTYON. 



body of the polypes, when we recollect that 

 their surface is covered with the stinging 

 organs. These lice are not, however, found 

 upon perfectly healthy polypes ; impurity of 

 the water and an unhealthy state being 

 generally denoted by their presence. 



BiBL. Leeuwenhoeck, Phil. Trans. 1/03, 

 xxiii.; Trembley,j:Tfm. s. l.Polyp. d'eau douce; 

 Ehrenberg, Cor all. d. roth. Meer. ; Laurent, 

 Rech. s. I'Hydre, Sf-c. ; Corda, Ann. d. Sc. 

 nat. 2 ser. viii. ; SchaefFer, Die Armpolypen ; 

 Erdl, Muller's Archiv, 1841 j Ecker, Siebold 

 and KolliJcer's Zeitschr. i. ; Johnston, Brit. 

 Zoophytes ; A. Thomson, Todd's Cycl. Anat. 

 and Phys. iv. p. 1 7 ; Hancock, Ann. Nat. Hist. 

 2ser. v.p.281.; AWman, Microsc.Journ. 1854. 



HYDRACHNA, Miill.— A genusof Arach- 

 nida, of the order Acarina and family 

 Hydi-achna. 



Char. Palpi tolerably long, third joint 

 longest, the fourth and hfth terminated each 

 by a claw ; mandibles ensiform ; rostrum 

 long, scarcely shorter than the palpi ; body 

 rounded ; eyes distant ; vulva concealed by 

 a plate or shield. 



When young, these little water- spiders 

 have three legs only, and in this state have 

 formed another genus, Achlysia. 



H. cruenta, Sliill. = H. globula, Herm. 

 (PI. 2. fig. 29). Body subovate; two pairs 

 of eyes at a moderate distance apart, reniform, 

 dark red ; skin covered with minute puncta. 



The rostrum is broad and curved at the 

 base (fig. 29 c, the lower part directed to the 

 left), cleft above, so as to form a kind of 

 channeled sheath, containing the anterior 

 narrower portions of the two mandibles [b). 

 The palpi (c, upper organ) are inserted upon 

 the sides of the base of the rostrum and 

 curved downwards ; the first joint is very 

 broad, the second much cm-ved, the third 

 long, and flattened on one side and rounded 

 on the other ; the fourth joint is short, and 

 terminated by a short and thick claw ; the 

 fifth also forms a claw, but the two claws do 

 not form a chela, their curves being parallel. 

 Of the legs (fig. 29 a), the three posterior 

 pairs are ciliated for swimming, and the 

 posterior are much longer than the anterior ; 

 the coxae are flattened and form two grou])s 

 on each side ; between the two posterior 

 coxae is the orifice of the reproductive organs; 

 the tarsi all have two claws, and are obliquely 

 truncated and concave at the ends (fig. 29 e). 



The eggs are reddish-brown and deposited 

 upon the stems of water-])lants; the nymphae 

 are found attached to aquatic insects (fig. 

 29/), as Nepa, Dyfiscus, &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. 



BiBL. Duges, Ann. d. Sc. nat. 2 ser. i. ; 

 Gervais, Walckenaer's Arachn. iii. ; Koch, 

 Deutschl. Crustac, ^c. 



HYDRIAS, 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. 34. fig. 39). Body ovate, 

 hyaline ; foot narrowed into the form of a 

 slightly forked tail; aquatic; length 1-190". 



Probably a young and imperfectly exa- 

 mined Philodina. Found in Egypt. 



BiBL. Ehrenberg, Infus. p. 483. 



HYDROCHARIDACE^.— A family of 

 Monocotyledonous Flowering Plants growing 

 in water, interesting to the microscopist as 

 afi^ording 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 oiAnacharisA Isinastrum, a N.American 

 plant, now naturalized in streams in many 

 parts of Britain, also show the circulation 

 well. The extremities of the roots of Hydro- 

 charis morsus-rance, a plant common, floating 

 on the surface, in broad, permanent 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 contain green globules of chlorophyll, 

 these mostly adhere to the circulating mass, 

 and are carried round with it. The phseno- 

 menon may be observed in uninjured young 

 leaves simply immersed in water, b)'^ focus- 

 ing carefully; 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 fragments 

 if they are kept wet. Sometimes it is arrested 

 by the preparation ; in such cases the appli- 

 cation of a gentle heat often causes it to re- 

 commence. It may be observed with a power 

 of 200 diameters, but a higher is requisite 

 for minute investigation. (See Rotation.) 



HYDRODICTYON, Roth.— A genus of 

 Siphonaceae (Confervoid Algae), containing 

 one species, H. utriculatum, found in fresh- 

 water pools in the midland and southern 



