OXYTRICHA. 



[ 485 ] PACINIAN CORPUSCLES. 



OXYTRICHA, Bory, Ehr. A genus 

 of Infusoria, of the family Oxytrichina. 



Char. Neither styles, hooks, nor horns 

 present. Ehrenberg describes eight species; 

 some are marine, others aquatic. 



O. pellionella, E. (PI. 24. fig. 52). Body 

 whitish, smooth, slightly depressed, equally 

 rounded at the ends, often somewhat broader 

 in the middle ; head not distinct ; mouth 

 ciliated ; tail with bristles. Aquatic ; length 

 1-720 to 1-280". 



0. ffibba, E. (PI. 24. fig. 53). Body white, 

 lanceolate, obtuse at each end, ventricose in 

 the middle ; ventral surface flat, with a dou- 

 ble row of seta3; mouth large, rounded. 

 Aquatic; length 1-240". 



Dujardin places his genus Oxytricha among 

 the Keronia, with the characters : body soft, 

 flexible, oval or oblong, more or less de- 

 pressed, with cirrhi or larger non-vibratile 

 cilia in the form of bristles or styles, but 

 without horns ; and describes nine species, 

 mostly not corresponding to those of Ehren- 



ic whole requires revision. 



BIBL. Ehrenberg, Infus. p. 363; Dujar- 

 din, In/us, p. 416. 



OXYTRICHINA, Ehr. A family of In- 

 fusoria. 



Char. Carapace absent; alimentary ori- 

 fices two, neither terminal ; body furnished 

 with vibratile cilia and bristles, non-vibratile 

 styles or hooks. 



Body depressed ; locomotive organs prin- 

 cipally situated upon the under surface. 

 Propagation by longitudinal and transverse 

 division, and by the periodical formation of 

 egg-like granules. 



The five genera are thus distinguished : 



Cilia and bristles present, but no styles nor hooks. 



No anterior horns Oxytricha. 



Anterior horns present Ceratidium. 



Cilia present, with styles or hooks, or both. 



Hooks present, but no styles Kerona. 



Styles present, no hooks Urostyla. 



Both styles and hooks present Stylonichia. 



BIBL. Ehrenberg, Infus. p. 362. 



OXYURIS, Rud. See ASCARIS. 



OYSTER (Ostrea).A genus of Lamelli- 

 branchiate Mollusca. 



The gills of O. edulis, the common oyster, 

 show the ciliary movement ; but it is not so 

 easily seen in this as in the marine mussel. 



The shells of the fry or ' embryo-oysters ' 

 exhibit the black cross and an imperfect set 

 of coloured rings with polarized light. 



P. 



PACHNOCYBE, Berk. A genus of Stil - 

 bacei (Hyphomycetous Fungi), somewhat 

 confused at present withDoromyces,Corda, 

 and Periconia, Nees. These plants have an 

 erect filiform stem, composed of conjoined 

 filaments, capitulate above, the head being 

 pruinose (not flocculent), with crowded sim- 

 ple spores. The pedicels are mostly brownish 

 or blackish, the spores light-coloured ; the 

 entire plants from 1-24 to 1-6" high. Seve- 

 ral species occur on rotten wood, stems, &c. 



BIBL. Berk. Hook. Brit. Flor. ii. pt. 2. 

 p. 333, Ann. Nat. Hist. 2 ser. v. p. 465; 

 Fries, Summa Veg. p. 467. 



PACHYGNATHUS, Duges. A genus of 

 Arachnida, of the order Acarina, and family 

 Trombidina. 



Char. Palpi conical, last joint scarcely 

 forming a claw ; mandibles stout, chelate ; 

 body entire, narrowed in front; coxae distant; 

 legs gressorial, sixth joint very long, seventh 

 very short; anterior legs longest and stoutest. 



P. velutinus (PI. 2. fig. 34), the only spe- 

 cies. Found in autumn, under damp stones. 

 The hairs covering the body are short, flat, 

 and curved, giving it a velvety aspect. Body 

 inflated, narrowed in front, the narrowed 

 portion with two projecting brownish eyes. 

 The insertions of the legs are in two groups, 

 not far distant from each other nor from the 

 median line ; second pair of legs shortest ; 

 in all the sixth joint is very long, the seventh 

 very short and narrow (6), as in Tetranychus, 

 Megamerus, and Raphignathus ; claws two, 

 large ; rostrum projecting ; palpi (a) short, 

 about twice the length of the labium ; man- 

 dibles chelate or like a lobster's claw, very 

 large and stout at the base. Movement 

 slow. 



BIBL. Duges, Ann. d. Sc. nat. 2nd ser. 

 ii. p. 54 ; Gervais, Walckenaer's Apter. iii. 

 p. 171. 



PACINIAN CORPUSCLES. These 

 curious organs are found as terminations of 

 the spinal nerves in the skin and subcuta- 

 neous tissue of the palm of the hand, the 

 sole of the foot, the fingers and toes, in the 

 sympathetic semilunar ganglia, the mesen- 

 tery, &c. 



They are elliptical or pear-shaped, whitish, 

 about 1-25 to l-6th in diameter. Each con- 

 sists of from twenty to sixty concentric 

 layers of areolar tissue (fig. 558), the inter- 

 spaces between the outer being considerable, 

 those between the inner being small, and 



