ACANTHACE.E. 



ACANTHOMETRINA. 



(PI. 49. fig. 10), from each of which eight 

 bipartite processes shoot out in a whorl 

 (Strobila-state) . The segments of the body 

 then separate from each other seriatim, from 

 before backwards, swim about with eight 

 rays (PI. 49. fig. 11), and at last become 

 gradually developed into perfect Medusas. 

 Many of the Medusae are phosphorescent, 

 and render the sea luminous. 



Gegenbaur divided a Thaumantias into a 

 hundred pieces, and found that each piece, 

 provided it contained a portion of the mar- 

 gin of the umbrella, grew into a perfect 

 small Medusa. 



Many of the organisms formerly con- 

 sidered Medusa are now regarded as the 

 Medusoid buds or gonozoids of the Hy- 

 droida. See THAUMANTIAS. 



A small Medusa, with 2 names : Craspeda- 

 custes SowerbyiandLimnocodium Victoria ,has 

 been found in the tropical fresh-water lily- 

 tank at the London Royal Botanic Gardens. 

 This ought to be the Medusoid gonophore 

 of a hydroid zoophyte ! 



BIBL. Eschscholtz, Syst. d. Acal. Berlin, 

 1829; Will, Hoi-ce Tergest. $c., 1844; 

 Ehrenberg, Abhandl. d. Berl. Akad. 1835 j 

 Art. Acal., Tcdd's Cycl (R.- Jones) ; Sie- 

 bold, Lehr. d. Vergl. An. ; Huxley, Inver- 

 tebrata ; Lesson, Suites a Buffon (Zooph. 

 Acalephes) ; Wagner, Icon. Zootom. ; Ge- 

 genbaur, Vergl. Anat. 1878 ; Gosse, Mar. 

 Zool. ; Forbes, Nak.-eyed Medusce (Ray 

 Soc.) ; Kolliker, Icon. Histol. 1865 ; Haeckel, 

 Syst. d. Medus., 1879 ; Romanes, Phil. Tr. 

 1876, p. 269 (Muse, syst.}; Nicholson, 

 Zool., 1878; Pascoe, Zool. Class., 1880; 

 Kowalewsky, Ann. Nat. Hist. 1867, xx. 228; 

 R. Lankester, Qu. Micr. Jn. 1881, p. 119. 



ACANTHA'CILE. The seeds of many 

 genera of this family are clothed with hairs 

 composed of hygroscopic cells, containing 

 unreliable spiral fibres or detached rings. 

 Among these are Acantliodium spicatum, De- 

 lile, Blepharis, and Ruellia formosa. Other 

 species and genera have the hygroscopic cells 

 destitute of internal fibre, as Ruellia littora- 

 lis, Phaylopsis glutinosa, Barleria noctiflora, 

 Lepidagathis, 8fc. Further particulars re- 

 specting the hygroscopic cells will be found 

 under CELL-MEMBRANE and SPIRAL STRUC- 

 TURES. See also ACANTHODIUM and 

 RUELLIA, and, for a similar phenomenon in 

 other families, COLLOMIA, COBJEA, SALVIA. 



BIBL. Kippist, Linn. Trans, xix. p. 65. 



ACANTHOCYS'TIS, Carter. A genus 

 of Rhizopocla, apparently referable to the 

 Actinophryina. 



Char. Rounded, green, with movealle 

 radiating spines and pseudopodia. Body 

 flexible, covered with minute fusiform 

 curved spicula ; spines straight, hollow, 

 bifid, discoid at base. 



A. turfacea (PI. 51. fig. 9). Found in 

 heath-bog water; diam. of body JTT"- 



BIBL. Carter, Ann. N. Hist. 18(33, xii. 

 p. 263 ; Hertwig, Jen. Zeitsch. (Qu. Mic. Jn. 

 1878, xviii. p. 205). 



ACANTHO'DIUM (Flowering Plants, 

 fam. Acanthacea). Kippist first described 

 the curious hairs upon the seed of Acantho- 

 dium spicatum, Delile (PI. 28. fig. 24). The 

 entire surface of the seed is clothed with 

 hairs of whitish colour, appressed and closely 

 adherent in the dry state, being apparently 

 glued together at their extremities. When 

 placed in water, the hairs are set free and 

 spread out on all sides ; they are then seen 

 to consist of clusters of from five to twenty 

 spiral cells firmly coherent below, but free 

 above and separating from the cluster at 

 different heights, expanding in all directions 

 like plumes, and forming a very beautiful 

 microscopic object. The free portions of 

 the cells elongate so as to separate the coils 

 of one, two, or occasionally three internal 

 spiral fibres, which are sometimes branched 

 and not unfrequently broken up into rings ; 

 at the lower part of the cells the turns of 

 the spiral are connected by perpendicular pro- 

 cesses so as to convert the spiral into a reticu- 

 lated structure. See SPIRAL STRUCTURES. 



BIBL. Linn. Trans, xix. p. 65. 



AC ANTHOME'TRA, Mull. A genus of 

 ACANTHOMETRINA. 



ACANTHOMETRI'NA. A family of 

 Radiolarian Rhizopoda. 



Char. Body minute, spherical, capsular ; 

 traversed by numerous elongate, mostly 

 angular and hollow siliceous spines, which 

 meet in the centre. Between the spines, 

 pseudopodia radiate from the body (PI. 51. 

 fig. 10), as in Actinophrys. Marine. 



The body contains yellow globules, and 

 is sometimes covered with small spicules; 

 and it is enveloped by a softer cortical sar- 

 ccdic mass. 



The Acanthometrina, with the Polycys- 

 tina, have been rearranged by Hackel, in 

 his splendidly illustrated monograph, into 

 68 genera and 150 species. 



They are found recent on the surface and 

 at the bottom of the sea, in the Mediterra- 

 nean, the Adriatic, and the North Sea. 



They form beautiful microscopic objects. 

 See RADIOLARIA. 



B 2 



