HORN. 



[ 326 ] 



HYALOSIRA. 



but of rows of individual cells imbedded in a 

 filiform gelatinous tube (fig. 340), analogous 



Fig. 340. 



Fig. 341. 



Hormospora transversalis. 



Fragments of gelatinous filaments, with the cells grouped 

 in fours. 



Magnified 350 diameters. 



in its nature to the gelatinous coat investing 

 the linear rows of cells of Hyalotheca, &c. 

 The cells multiply by transverse division, 

 the rows thus becoming elongated; these 

 cells contain green contents arranged in a 

 granular, lamellar, or radiating form. Bre- 

 bisson describes obscurely another mode of 

 increase, in which the "endochrome becomes 

 concentrated and organized into vesicles or 

 zoospores. The corpuscles then become 

 larger, and the filament becoming as it were 

 dislocated, the corpuscles group themselves 

 in several rows, and without regular form " 

 (fig. 341). In H. transversalis there is an 

 especial tendency to a grouping of the cells 

 in fours. Three species have been described; 

 1 and 3 are known as British. 



1. H. mutabilis, Breb. Filaments simple; 

 cells ovoid or subspherical ; cell-contents 

 lamellar. Aquatic. Breb. Ann. des Sc. nat. 

 3ser. i. pl.l. fig. 1. 



2. H. transversalis, BreT>. (figs. 340,341). 

 Filaments simple; cells ovoid or fusiform, 

 transverse; contents granular. Aquatic. Breb. 

 I. c. fig. 2. 



3. H. ramosa, Thwaites. Filaments 

 branched ; cells oval or spherical ; contents 

 radiated. In a pool to which salt water had 

 access. Harvey, PJiyc. Brit. pi. 21 3. 



BIBL. Brebisson, Ann. des Sc. nat. 3 ser. 

 1; Harvey, Brit. Mar. Alg. p. 235. pi. 27 B, 

 Phyc. Brit. pi. 213; mge^Einzell.Alg. p. 7. 

 pi. 3. fig. B. 



HORN. The horns of animals are of 

 three kinds ; those composed of bone, those 

 consisting of epidermic formations, and 

 those in which both are present. The former, 

 properly called antlers, agree in minute struc- 

 ture with bone, and therefore require no 

 special notice. The horn of the rhinoceros 

 may be taken to represent the structure of 

 the second kind. It consists of an aggrega- 



tion of horny fibres, each of which is made 

 up of a series of concentric layers. These 

 layers are composed of cells tangentially flat- 

 tened, and sometimes containing pigment. 

 The cells may be separated by macerating 

 the horn in solution of potash. Cracks 

 filled with air are frequently visible between 

 the layers. The centres around which the 

 laminae are arranged, probably correspond to 

 papillae of the cutis. 



The horn of the buffalo agrees essentially 

 in structure with that of the rhinoceros. 



The third kind of horn is exemplified by 

 that of the cow. In its centre is a process 

 of bone, surrounding and extending beyond 

 which is the proper horn, consisting of con- 

 centric layers, in the natural state composed 

 of flattened, irregular, angular nucleated cells 

 (PL 17. fig. 29 a), which assume their primi- 

 tive forms under the action of potash (6) ; 

 some of them contain pigment (c). Be- 

 tween the laminae cracks containing air are 

 also met with (f). 



Sections of horn made at various angles to 

 the axis form very beautiful polarizing ob- 

 jects; the gorgeous colours seen in those of 

 rhinoceros's horn cannot be excelled, nor can 

 drawings represent them faithfully (PI. 31. 

 figs. 37, 38). The horn of the buffalo also 

 forms an interesting object of the same 

 kind. 



BIBL. Bonders, Mulder's Physiolog. Che- 

 mie ; Owen, Brande's Diet., Art. Cornua. 



HORSE-LEECH. See H^MOPIS. 



HYALODISCUS, Ehr. = CvcLOTELLA, 

 Kiitz. in part. 



H. Ifsvis = Cyclotella Itevis. 



H. patagonica = Cycl. patagon. 



BIBL. Ehrenberg, Ber. d. Berl. Akad. 

 1845, pp. 78 & 155; Kiitzing, Sp. Alg. p. 20. 



HYALOSIRA, Kiitz. A genus of Diato- 

 maceee. 



Char. Frustules concatenate, rectangular, 

 tabulate; with alternate vittae, interrupted 

 in the middle, and connected with those of 

 the opposite side by fine lines ; lowermost 

 frustule attached by a stipes which is affixed 

 to one angle. Marine. 



The fine lines at the end of the vittae give 

 the latter a forked appearance. The frus- 

 tules are often partly separated, so as to be 

 connected with each other by one angle only. 



Four species, probably not really distinct. 



H. rectangula (PI. 13. fig. 1). Stipes 

 short, frustules subconcatenate, in front view 

 subquadrate, rectangular; length 1-1380". 



BIBL. Kiitzing, Bacillar. p. 125; Sp. 

 Alg. p. 115. 



