THE VOYAGE OF H.M.S. CHALLENGER. 



yellow either with nitric acid or with iodine. In dilute mineral acids they dissolve more rapidly 

 than in concentrated. My usual method of cleansing the skeleton of ACANTHARIA (which has 

 been practised with the same result on thousands of specimens) consists in heating the preparation 

 in a small volume of concentrated sulphuric acid and then adding a drop of fuming nitric acid ; all 

 other constituents (the whole central capsule and the calymma) are thus very rapidly destroyed ; 

 the skeleton remains quite uninjured and withstands the combined action of the mineral acids for 

 a longer or shorter time, though on prolonged heating it also is dissolved. I do not therefore 

 regard acanthin as an albuminous substance, but as one related to chitin. 



D. Calcareous skeletons have not been certainly demonstrated in the Eadiolaria, and probably 

 do not occur. Sir Wyville Thomson in his Atlantic (1877, L. N. 31, vol. i. p. 233, fig. 51) 

 described under the name Calcaromma calcarea, a Eadiolarian which contained scattered in its 

 calymma numerous calcareous corpuscles " resembling the rowels of spurs." These are identical 

 with the "toothed bodies, recalling crystal balls," which Johannes Miiller figured in the 

 Mediterranean Thalassicolla morum so early as 1858, and compared with the " siliceous asterisks of 

 Tethya" (L. K 12, p. 28, Taf. vii. figs. 1, 2). I formerly regarded these peculiar calcareous 

 corpuscles, whose solubility in mineral acids I had observed, as spicules of a Thalassicollid, and hence 

 described the species in my Monograph as Thalassosphcera morum (L. N. 16, p. 260). I have, 

 however, seen reason to change my view, and am now led to suppose that those peculiar calcareous 

 corpuscles, which may be named " Calcastrella," are not formed by the Eadiolarian itself, but are foreign 

 bodies which have been accidentally incorporated into the calymma of a Thalassicollid (Actissa). 

 These corpuscles occur, often in large numbers, in many preparations in the Challenger collection, 

 and in the calymma of other Eadiolaria, chiefly Discoidea, hence it would appear that they 

 are foreign bodies taken up by the pseudopodia and carried into the calymma by the circulation of 

 the sarcode. The Eadiolaria which Sir Wyville Thomson figured as Calcaromma calcarea, and 

 Miiller as Thalassicolla morum, I regard as species of Aciissa (see p. 13), perhaps Actissa radiata of 

 the Pacific, and Actissa primordialis of the Mediterranean (compare the description of the 

 Thalassosphserida of the Challenger collection, pp. 30, 31). 



103. The Physical Properties of the Skeleton. The skeletons of all Eadiolaria are 

 characterised pre-eminently by a high degree of firmness, which fits them to serve as 

 protective and supporting apparatus. This is obvious in the case of the pure siliceous 

 shells of the Polycystina ; but the acanthin framework of the ACANTHARIA also possesses 

 a degree of stiffness but little inferior, whilst the silicate skeletons of the PH^EODARIA 

 seem on the whole to be not so firm. The hollow skeletal tubes of the last-named, which 

 are filled with gelatinous material, are very brittle on account of the delicacy of their 

 walls. Their elasticity also is very small, whilst that of the acanthin spines is consider- 

 able. The thin long needles of many ACANTHARIA are very elastic, as are also the 

 bristle-like siliceous spicules of many SPUMELLARIA. The refractive power of the skeleton 

 in the various legions is very different, depending upon the chemical constitution. The 

 siliceous skeleton of the Polycystina (SPUMELLARIA and NASSELLARIA) and the silicate 

 skeleton of the PH^EODARIA have the same refractive index as glycerine, and hence become 

 invisible when mounted in that fluid; they then become visible only on addition of 



