IIORMIDIUM. 



[ 305 J 



HORN. 



the psoudo-thorax ; cephalotliorax articu- 

 laicd to the abdomen, ibldiug upon the 



bodv. 



Two species ; on decaying fii--wood and 



roots. 



BiBL. GeiTais, iralclrnaer's Aplcrcs, lu. ; 

 Claparede, Zeit. wiss. Zool. xviii. ; Murray, 

 Ec. Eni. 2i>2; Jn. Mic. Soc. 1880,177. 



HOliMID'lUM.— The group of species 

 of Ulothriv gTO\A-iug upou moist earth. 



HOKMIS'CIA, Aresch.— A genus of 

 Confervoid Algffi, close to Ulothrix ; with 

 the tilaments jointed, usually simple, but 

 sometimes emitting ramules ; cells with 

 thick, often lamellar walls ; propagation by 

 macro- and micro-gonidia. i'resh and salt 

 water. (Habenh. Fl. Ah/, iii. 361.) 



HORMOS'PORA, Breb.— A genus of Pal- 

 meUaceae (Confervoid Algse), A,Aith a frond 



Fig. 336. 



Fig. 337. 



Hormospora transversalis. 



Fragments of gelatinous filaments, with the cells 

 grouped in fours. 



Magnified 350 diameters. 



consisting of simple or branched gelatinous 

 confervoid cords enclosing rows of oval or 

 spherical cells ; they are found floating 

 among Confervas or other aquatic plants, 

 and appear to the naked eye lilie greenish 

 filaments. These plants do not appear to 

 consist of septate fliamentous tubes like the 

 ConfervEe, but of rows of individual cells 

 imbedded in a filifurm gelatinous tube 

 (fig. 336), analogous 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 becom- 

 ing elongated; these cells contain green 

 contents arranged in a granular, lamellar, 

 or radiating form. Brebisson describes ob- 

 scurely another mode of increase, in which 

 the " endochrome becomes concentrated and 

 organized into vesicles or zoospores. The 

 corpuscles then become larger; and the fila- 

 ment becoming as it were dislocated, the 

 corpuscles group themselves in several rows, 

 and without regular form " (fig. 337). In 



H. transvermlis there is an especial tendency 

 to a grouping of the cells in fours. Five 

 species have been described ; 1 and 3 are 

 known as British. 



JI. Dtutahilii^, Breb. Filaments simple ; 

 cells ovoid or subspherical ; cell-contents 

 lamellar ; freshwater. Breb. A)ui. Sc. Nat. 

 3ser. i. ph 1. tig. 1. 



IL transversalis, Breb. (figs. 336, 3.37). 

 Filaments simple ; cells ovoid or fusiform, 

 transverse ; contents granular ; freshwater. 

 Breb. I. c. fig. 2. 



H. ramosa, Thwaites. Filaments 

 branched ; cells oval or spherical ; contents 

 radiated. In a pool to which salt water 

 had access. Harvey, Phyc. Brit. pi. 213. 



BiBL. Brebisson, Ann. Sc. Nat. 3 ser. i.; 

 Harvev, JJr. Mar. Aly. 235, pi. 27 B, Phyc. 

 Brit. pi. 213 ; Niigeli, Einzell. Aly. 7, pi. 3. 

 fig. B ; Rabenhorst, Fl. Aly. iii. 48. 



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 

 structure with bone, and therefore require 

 no special notice. The horn of the rhino- 

 ceros may be taken to represent the structure 

 of the second kind. It consists of an aggre- 

 gation of horny fibres, each of which is made 

 up of a series of concentric laj'ers. 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 laj'ers. The centres around which the 

 laminas are arranged probably correspond to 

 papillas of the cutis. The horn of the 

 bufl'alo 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 (PI. 22. fig. 29 o), which assume their 

 primitive forms under the action of potash 

 (6) ; some of them contain pigment (d). 

 Between the laminte, cracks containing air 

 are also met with (/"). 



Sections of horn maile at various angles to 

 the axis, form very beautifid polarizing ob- 

 jects ; the gorgeous colours seen in those of 

 rhinoceros's horn cannot be excelled, nor can 

 drawings represent them faithfuUv (PL .30. 

 figs. 37, 38). The horn of the bufthlo 



