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MICROGRAPHIC DICTIONARY. 



Aberration.— The deviation of the 



rajs of lijrht from the true focus of a lens or 

 cui'ved mirror, in consequence of which 

 they do not unite at a single point, but 

 form an indistinct or coloured image of an 

 object. It arises from two causes : the 

 form of the lens or mirror, when it is called 

 spherical aberration ; and the different re- 

 frangibihty of the rays of hght, when it is 

 called chromatic aberration. 



ABROTHAL'LUS, Notaris and Tulasne. 

 — A genus of Lichens, Fam. Lichenacei, 

 remarkable for their parasitic habit and the 

 absence of a thallus. The species are now 

 referred to Lecidea. 



A. Smithii (Welwitschii and microsper- 

 mus) = L. parineliamm. 



A. oxi/spori's = L. oxysporus. 



A. inquinans = L. inquiiians. 



BiBLiOGEAPHY. Lindsay, Brit.Lich. 311 ; 

 Tidasue, Ann. Sc. Nat., Bot. xvii. p. 112, 

 1852 ; De Notaris, Mem. R.Acad. Sc .Turin, 

 X. p. 3-")l, 1859; Berkeley, Cryp. Bot. 405 ; 

 Leighton, Lichen-Flora, p. 384, 1879. 



ACALETHJE (Medusa)).— An order of 

 Coelenterata, commonly Imown as Sea-net- 

 tles, on account of their producing urtica- 

 tion when touched ; or Jelly-fishes, or Sea- 

 blubbers, from their gelatinous consistence. 



They consist of a transparent, floating 

 and free, discoid or spheroid body (hydro- 

 soma), often shaped like an umbrella ; and 

 vary in size from a mere speck to a yard in 

 diameter. Margin of the disk furnished 

 with filiform tentacles, cirri, &c. The 

 organs are radiate aroimd a longitudinal 

 axis, occupied by a central peduncle or 

 stalk (manubriuni), at the bottom of which 

 is the mouth. The disposition of the parts 

 is generally quaternary. 



The cutaneous surface of the body, or 

 ectoderm, is covered Avith a very dehcate 

 epidermis (PI. 49. tig. 2). Cilia exist on 

 various parts of the body, especially the 



Fig. 1. 



Thaumantias hemisphaerica, magnified 2 diameters. 



arms, tentacles, cirri, &c ; upon which also 

 peculiar stinging organs and organs of ad- 

 hesion occm*. In those species which are 

 notorious for their urticating powers, these 

 organs are also situated in aggregations be- 

 neath the epidermis of the body. The 

 stinging organs, or nematocysts, usually form 

 oval capsules, in which a spirally coiled 

 filament is enclosed (PI. 49. fig. 3 a, b) ; 

 this flies out on the slightest touch, with 

 the capsule to which it is attached, from the 

 irritated part of the slrin (I'l. 49. fig. 3 c). 

 In some Acalephpe, these stinging organs 

 are replaced by oval capsules from which a 

 rigid bristle projects (PI. 49. fig. 4). These 

 do not produce urtication, but enable the 

 animal to adhere to other bodies. Near 

 the surface of the body and between the 

 cells composing its substance, pigment-cells 

 frequently occur, some of which are isolated, . 



39 79.'-) 



