HALACARUS. 



[ 381 ] 



HALIONYX. 



scandens, the spiral-fibrous hairs take rather 

 the form of minute scales, and they do not 

 spontaneously expand elastically (PL 28. 

 fig. 20). Among the Compositse, these 

 spiral-fibrous hairs have been observed on 

 the pericarp of Ituckeria, some species of 

 Trichocline, Euriops, Mesogramma, Doria 

 Cluyticefolia, Oligothrix gracilis, and some 

 species of Senecio. Spiral cells also occur 

 on the seed of Hydrocharis. The best way 

 to observe the elastically expanding hairs is 

 to place a thin slice of the skin of the seed 

 on a slide in a little alcohol, which does not 

 soften the cell-wall : when the object is in 

 focus, the addition of a little water causes 

 the gelatinous softening of the cell-walls, 

 the spiral fibres fly out from the surface of 

 the seed-coat and show clearly the charac- 

 ter of these beautiful objects. The primary 

 membrane may be detected, even in its 

 gelatinous state, by adding sulphuric acid 

 and iodine, which produce a purplish or 

 violet colour. Further remarks on this head 

 will be found under SPIRAL, STRUCTURES. 

 The hairs on the stigma of Campanula 

 are remarkable for the intussusception which 

 is observed to take place in the mature hairs. 

 The filiform processes growing from the 

 under surface of the frondose Hepaticae, 

 the thallus of Lichens, the prothallium of 

 Ferns, &c., are commonly called radical 

 hairs. In most cases they present no re- 

 markable points of structure ; in Marchan- 

 tia, however, peculiar spiral markings have 

 been detected (see MARCHANTIA). 



BIBL. Meyen, Secretions-orc/ane d. Pflan- 

 zen, 1837 ; id. Pflanzen-physiol. ; Cohn, Cu- 

 ticula, Linncea, xxiii. 337, 1850 ; Schleiden, 

 Mutter' 8 Archiv, 1838; Beitr. z. Botanik, 

 Leipsic, 1844, i. 121 (Sc. Memoirs)-, De- 

 caisne, Arm. Sc. Nat. 2 ser. xii. 251, pi. 4 ; 

 Leighton, Ann. N. H. vi. 257 ; Brongniart, 

 Ann. Sc. Nat. 2 s<r. xii. 244, pi. 4; Pril- 

 lieux, Ann. Sc. Nat. 4 ser. v. 5; Tuffen 

 West, Qu. Mic. Jn. vii. 22; Weisse, D. 

 Pflanzenhaare, 1867; Hanstein, Bot. Zeit. 

 1868, 697; Kauter, Trichomgeb. 1871; 

 Martinet, Ann. Sc. Nat. 1872, xiv. ; Sachs, 

 Bot. 163. 



HALAC'ARUS, Gosse. A genus of 

 marine Acarina. 



Char. Body sometimes covered with a 

 dorsal shield ; rostrum bulbous, pointed ; 

 palpi terminated by a fang-like claw ; legs 

 formed for walking, arising from the outer 

 margin of the body, directed two pairs for- 

 wards and two backwards, and with a pair 

 of hooks. 



Five species : found crawling upon sea- 

 weeds at low water, or in dredgings. 



BIBL. Gosse, Mar. Zool. i. 177 ; Murray, 

 Econ. Entom. 205 (figs.). 



HALARACH'NE, Allman. A genus of 

 marine Acarina ; family Gamasea. 



Body elongate subcylindrical, with an 

 anterior dorsal plate. 



H. halichcBri. In the posterior nares of a 

 seal (Halichwrus gryphus) ; length 1-8". 



BIBL. Allman, Ann. N. H. 1847, xx. 

 47 ; Murray, Econ. Ent. 167 (fio-a.). 



HALE'CILDvE, Hincks. A family of 

 Hydroid Zoophytes. 



Gen. : Halecium and Ophiodes. 

 HALE'CIUM, Oken. A genus of Hy- 

 droid Zoophytes, family Sertulariidaa. 



Distinguished by the plant-like polypi- 

 dom, the stem consisting of numerous 

 parallel capillary tubes, and the cup-like 

 nearly sessile cells arising alternately on 

 opposite sides of the stem, one under each 

 joint. 



H. halecinum. Vesicles oval or oblong. 

 Common on shells and stones in deep water ; 

 4-10" high. 



H. Beanii. Vesicles calceoliform. Rare. 

 H. muricatum. Vesicles spinous. 

 5 other species. 



BIBL. Johnston, Brit. Zooph. 58 ; Hincks, 

 Hydroid Zooph. 220. 



HALIOHOND'RIA, Flem. A genus of 

 SPONGES. 



HAL'IDRYS, Lyngb. A genus of Fu- 

 caceae (Fucoid Algae), containing one British 

 species, If. siliquosa, common on rocks and 

 stones somewhat above low-water mark. 

 It is readily distinguished by its pod-like 

 septate air-vessels. The fructification, which 

 is terminal on the branches, much resembles 

 that of Fucus, except that the interior 

 of the receptacles is filled up with firm 

 polygonal cellular tissue. The antheridia, 

 moreover, are terminal on their pedicels, 

 often in tufts, short in form, and inter- 

 mixed with spore-sacs in the same concep- 

 tacle. 



BIBL. Harvey, Br. Mar. Alg. 15, pi. 1 C ; 

 Thuret, Ann. Sc. Nat. 3 ser. xvi. 8, pi. 3 



HALIO'NYX, Ehr.-A genus of Diato- 

 maceae. 



Char. Frustules single; valves equal, 

 circular, surface radiate, the rays not com- 

 mencing at the umbilicus; no internal septa. 

 Marine. 



H. senarius. Rays six, the intervening 

 spaces with shorter rays of equal length 

 parallel to the larger, and with transverse 



