ISTHMIA. 



[ 370 ] 



IXODES. 



The archegonium is of mucli the same cha- 

 racter, essentially, as that of the rest of the 

 higher Ci-j^togamous Plants, consisting of 

 a ]3apilla with a central canal leading to the 

 embryo-sac. The four rows of cells forming 

 the neck of the archegone separate, and a 

 germ-cell is formed in the embryo-sac. This 

 is fertilized by the entrance of a spermato- 

 zoid into the embryo-sac. 



In the development of the embryo in the 

 spore, it forms a cellular body, which gra- 

 dually displaces the cellular tissue originally 

 filling this up. The first leaf and roots are 

 developed while the rudiment is still within 

 the spore-coat, in opposite directions and 

 horizontally (right and left) in relation to 

 the apex of the spore. The young plant 

 somewhat resembles a germinating Mono- 

 cotyledon. 



The woody structure of the stem of Isoetes 

 consists of spiral-fibrous cells, usually annular 

 or reticulated, but sometimes really spiral. 



BiBL. Bischofi^, Crypt. Gewachse, Rhizo- 

 carpen, Nuremberg,1828. p. 70; Mohl, Verm. 

 Schrift. Tubingen, 1845. p. 122 ; Miiller, 

 Botanische Zeit. vi. p. 297. 1848 [Ann. Nat. 

 Hist. 2 ser. ii. p. 81, &c.) ; Mettenius, Beitr. 

 z. Botanik. 1st heft, Heidelberg, 1850; 

 Hofmeister, Abhand. d. K. Sachs. Ges. d. 

 Wiss. iv. 123; A. Braun, Flora, 1847- p. 

 33. 



ISTHMIA, Ag. — A genus of Diatomacese. 



Char. Frustules depressed or subcylin- 

 drical, rhomboidal or trapezoidal in front 

 view, angles more or less produced ; frus- 

 tules coherent by the angles, basal frustule 

 stipitate ; surface of valves and hoop appear- 

 ing reticular or cellular. Marine. 



The depressions upon the valves and hoop 

 are so large as to produce a distinct reticular 

 or cellular appearance when viewed by ordi- 

 nary illumination. 



I. obliquata {nervosa, K.). Valves fur- 

 nished with linear thickenings, giving them 

 a coarsely reticular or veined appearance. 

 British. 



J. enervis (PI. 13. fig. 2). Valves without 

 linear thickenings, uniformly covered with 

 depressions. British. 



BiBL. Ehrenberg, Die Infus. p. 209 ; 

 Kiitzing, Bacill. p. 137, and Sp. Alg. p. 135 ; 

 Ralfs, Ann. Nat. Hist. 1843. xii. p. 270. 



ITCH-INSECT. See Sarcoptes. 



IVORY. — This substance, which consists 

 of the tusks of the elephant, possesses the 

 minute structure of bone. 



IVORY, Vegetable. — This substance, 

 consisting of the albumen of the seeds of a 



Monocotyledonous tree, Phytelephas macro- 

 carpa, is composed of cellular tissue, w ith the 

 w^alls so thickened by horny secondary deposits 

 that the cavities of the cells are almost oblite- 

 rated. The pores of the secondary deposits, 

 however, remain uncovered throughout all the 

 thickening, and thus are converted into tubes 

 or canals running to meet each other from 

 the small remaining cavities of contiguous 

 cells. In PL 38. fig. 23 c represents a sec- 

 tion mounted in Canada balsam, which has 

 in part penetrated into the cavities ; the re- 

 maining cavities and pore canals are filled 

 with air and thus appear black {a). 



IXODES, Latr. — A genus of Arachnida, 

 of the order Acarina, and family Ixodei. 



Char. Palpi canahculate, sheathing the 

 rostrum; mandibles 3-jointed, basal joint 

 internal, the second joint external and long, 

 the third short, denticulate ; labium covered 

 with reflexed teeth ; body very extensile, fur- 

 nished near the rostrum with a dorsal horny 

 shield ; legs \^dth two claws and a caruncle. 



These animals form part of those which 

 are popularly known as ticks. They are 

 commonly found in dense woods, upon 

 brushwood, briers, &c., from which they get 

 upon animals, as dogs, oxen, horses, &c., 

 burying the rostrum deeply in the skin 

 and sucking the blood, so as to become dis- 

 tended to ten times their original size. They 

 are also found upon reptiles, birds, and occa- 

 sionally attack man. 



The species are very numerous, and have 

 been arranged in several genera by some 

 authors. 



I. ricinus, the dog -tick. Body oval, in 

 the gorged condition becoming globular and 

 blackish violet; legs and appendages brown. 



/. reduvius. Pale yellowish red ; head 

 and legs black. Found upon sheep. 



I. pictus. Back white, with brown spots j 

 crenulate posteriorly; legs brown. Found 

 upon deer ; also upon mosses. 



/. Dugesii [plumbeus. Dug.) (PI. 2. figs. 

 19-22). Oval, leaden grey, without spots. 

 Found upon dogs. 



/. plumbeus. Leach. Shield heart-shaped, 

 slightly rugose ; rostrum, palpi and legs 

 pale ferruginous ; body of a leaden colour ; 

 length 1-4". Found upon and in the nests 

 of the bank-swallow {Hirundo riparia). 



BiBL. Gervais, Walckenaer's Apteres, in. 

 p. 234; Hermann, Mem. Apterolog .; Duges, 

 Ann. d. Sc. nat. 2nd ser. ii. ; Leach, Linn. 

 Trans, xi. ; Koch, Uebers. d. Arachnid. 

 Syst. ; Denny, Ann. Nat. Hist. 1843. xii. ; 

 Gene, ibid, 1846. xviii. p. 160. 



