REPORT ON THE RADIOLAEUA. Ixxi 



water, and are clearer in proportion to the quantity of water which is added. The 

 refractive index of acanthin is, however, very different from that of glycerine, so that 

 the skeletons of ACANTHARIA are readily visible when mounted in this fluid. In water, 

 the skeletons of all Radiolaria appear about equally refractive, as also in Canada balsam. 

 The substance of the skeleton appears almost entirely hyaline, colourless, and transparent. 

 Very rarely it is faintly coloured (in some ACANTHARIA). A cloudy opaque constitution 

 is seen in some PH^EODARIA (especially in the " porcellanous shells " of Tuscarorida and 

 Circoporida, Pis. 100, 114-117); when dried, these appear by reflected light milky- 

 white or yellowish-white ; the cause of this opacity lies partly in the peculiar " cement- 

 like structure " of these porcellanous shells, partly in their fine porosity, and the minute 

 air-bubbles contained in their thick walls. 



104. The Elementary Structure of the Skeleton. The general constitution of the 

 skeleton or more accurately expressed, of the morphological elements of which the 

 skeleton consists is of such a nature that it may be termed structureless. Both the 

 organic acanthin skeletons of the ACANTHARIA and the silicate skeletons of the 

 PH^EODARIA, as well as the inorganic siliceous skeletons of the SPUMELLARIA and NASSEL- 

 LARIA, appear under the microscope perfectly homogeneous, transparent, colourless, and 

 crystalline. Only very rarely do they show traces of a concentric striation, which arises 

 from the deposition of the skeletal substance in layers ; as, for example, the thick spines 

 of some PH^ODARIA (Pis. 105-107, &c.). Some of the PH^ODARIA, however, form an 

 exception to this rule, inasmuch as their partially tubular skeletal elements possess a 

 remarkable porcellanous structure. In the tubular or Cannoid skeleton, which occurs in 

 most CANNOPYLEA, the lumen of the thin- walled flinty tube is filled with jelly, and 

 frequently a thin siliceous thread runs in its axis, and is connected with the wall by 

 transverse threads ( 127, 139). The elementary structure of the opaque porcellanous 

 shells, which distinguish the two families Circoporida (Pis. 114-117) and Tuscarorida 

 (PL 100), is quite peculiar. Numerous fine siliceous spicules lie scattered irregularly in 

 a finely granular or porous matrix. 



105. Complete and Incomplete Lattice-Shells. In the great majority of Radiolaria 

 (in all four legions) the skeleton has the form of a delicate lattice-shell or a receptacle in 

 which the central capsule is enclosed. In a small minority, however, this is not the case. 

 The skeleton then consists only of isolated rigid pieces (radial or tangential spicules), or 

 of a simple ring (sagittal ring of the S t e p h o i d e a), or of a basal tripod with or without 

 a loose tissue of trabeculse, &c. (Plectoidea); the central capsule is then not sur- 

 rounded by a special latticed receptacle, but only rests upon the skeletal trabeculse. 

 According to these different arrangements, two principal groups or sublegions may be dis- 

 tinguished in each legion, of which one set (Cataphracta) are characterised by a complete 



