ARACHNOIDISCUS. 



[ 59 ] 



ARCELLA. 



addition of acetic acid, showing the fibres of 

 elastic tissue. 



Fig. 31. 



Areolar and elastic tissue of arachnoid, after treatment 

 with acetic acid. Magnified 350 diameters. 



BIBL. Kolliker, Mikroskop. Anatomic, ii.; 

 Henle, Allgemeine Anat. 



ARACHNOIDISCUS, Bailey. A genus 

 of Diatomaceae. 



Char. Frustules disk-shaped, single, ad- 

 herent ; valves circular, flat, or very slightly 

 convex, exhibiting (even under a low power) 

 concentric and radiating lines, with appa- 

 rently cellular markings occupying the inter- 

 spaces, and a central clear space (pseudo- 

 nodule). 



The cellular appearance arises from the 

 existence of the ordinary depressions. Ehren- 

 berg mentions imperfect septa. 



This genus corresponds to the Hemiptychus 

 of Ehrenberg, who observes that as the term 

 Hemiptycha has been previously applied to 

 a genus of Hemiptera, that of Arachnoidiscus 

 will be preferable. 



A . Ehrenbergii, Bail. Pseudo-nodule sur- 

 rounded by an inner ring of linear radiating 

 and an outer ring of circular or angular 

 markings (depressions) ; marine ; breadth 

 1-200 to 1-60"; occurs also in guano (PI. 

 12. figs. 12 & 13, side view). 



A. ornatus, Ehr. South America. 



BIBL. Ehr. Eer. d. Eerl. Akad. 1848 & 

 1849; Smith, Brit. Diat.-, Shadbolt, Trans. 

 Micr. Soc. iii. 



ARANEA, Latr. (Tegenaria, Walcken.) A 

 genus of Arachnida, of the Order Araneidea. 



A. civilis and domestica, house-spiders; 

 readily accessible for examining the struc- 

 tural peculiarities of spiders, the organs of 

 the mouth, maxillary palpi, spinnerets, legs, 

 &c. See PL 2. 



BIBL. Walcken. Apteres, i. ; Koch, Die 

 Arachn. viii. ; Treviranus, Ueb. d. inn. Bau 

 d. Arachn. 



ARAUCARIA, Jussieu. A genus of Co- 

 niferae (Gymnospermous Flowering Plants), 

 remarkable for the character of the markings 

 on the walls of the cells of the wood, where 

 the disks or bordered pores appear in two or 

 more parallel rows (PL 39. fig. 5). Arau- 

 caria (Eutassa] excelsa is the Norfolk Island 

 Pine, which grows to an immense size, as do 

 also A. brasttiensis, A. imbricata, &c. The 

 reservoirs of turpentine seem to be in the 

 bark and not in the wood. See CONIFERS, 

 WOOD, and SECONDARY LAYERS. 



ARAUCARITES, Goeppert (Dadoxylon, 

 Endlicher ; Pinites, Lindl. and Hutt.). A 

 genus of Fossil Conifers (Gymnospermous 

 Flow. Plants), characterized by the many 

 rows of disks on the walls of the wood-cells. 

 Mostly occurring in the carboniferous forma- 

 tions. See CONIFERS. 



BIBL. Witham, Internal Structure of Fos- 

 sil Vegetables, p. 72, pi. 4-11, Edinb. 1833; 

 Lindley and Hutton, Fossil Flora, 1. t. 2, 3. 



ARCELLA, Ehr. A genus of Infusoria, 

 of the family Arcellina. 



Char. Animals contained in a carapace, 

 from an orifice in which one or more variable 

 flattened expansions are protruded ; carapace 

 discoid or hemispherical, with the orifice 

 upon the flat surface. 



The Arcellae correspond to Amoebae contained 

 in a carapace. In some species, the carapace 

 is membranous and uniform ; in others, it is 

 siliceous and exhibits fine stria3, depressions, 

 or granules (?) spirally arranged. Not un- 

 frequently it contains particles of sand, mi- 

 nute Diatomaceae, &c., imbedded in its sub- 

 stance. The gastric cells and contractile 

 vesicle of the Infusoria have been observed. 



Dujardin places this genus among the 

 RHIZOPODA. The species (?) are numerous ; 

 the most common are 



1. A. vulgaris (PL 23. fig. 14 a). Carapace 

 brownish-yellow, plano-convex, or hemi- 

 spherical, covered with depressions. These 

 markings or depressions are very beautiful 



