CXXXiv THE VOYAGE OF H.M.S, CHALLENGER. 



become united by connecting threads, the circulation of the granules may proceed quite 

 irregularly in the network thus formed. The rapidity and character of the extracapsular 

 currents are subject to great variations. 



The different forms of extracapsular sarcode currents have been already very fully described 

 in my Monograph (L. N. 16, pp. 89-126), and in my critical essay on the sarcode body of the 

 Ehizopoda (L. N. 19, p. 357, Taf. XXVI.). 



210. Secretion. Under the name secretions, in the strict sense, all the skeletal 

 formations of the Radiolaria may be included. They may be divided according to 

 their chemical composition into three different groups : pure silica in the SPUMELLARIA 

 ,and NASSELLARIA, a silicate of carbon in the PH^ODARIA, and acanthin in the ACAN- 

 THARIA (compare 102). It may indeed be assumed that these skeletons arise directly 

 by a chemical metamorphosis (silicification, acanthinosis, &c.) of the pseudopodia and 

 protoplasmic network; and this view seems especially justified in the case of the Astroid 

 skeleton of the ACANTHARIA (114), the Spongoid skeleton of the SPUMELLARIA ( 126), 

 the Plectoid skeleton of the NASSELLARIA ( 125), the Cannoid skeleton of the PH^ODARIA 

 { 127), and several other types. On closer investigation, however, it appears yet more 

 probable that the skeleton does not -arise by direct chemical metamorphosis of the 

 protoplasm, but by secretion from it ; for when the dissolved skeletal material (silica, 

 acanthin) passes from the fluid into the solid state, it does not appear as imbedded in 

 the plasma, but as deposited from it. However, it must be borne in mind that a hard 

 line of demarcation can scarcely, if at all, be drawn between these two processes. In 

 the ACANTHARIA the intracapsular sarcode is the original organ of secretion of the 

 skeleton ; in the other three legions, on the other hand, the extracapsulum performs this 

 function ( 106, 107). In addition to the skeleton, we may regard as secretions (or 

 excretions) the intracapsular crystals (75) and concretions ( 7 5 A), and perhaps certain 

 pigment-bodies ( 74, 88) ; and further, the calymma ( 82) may be considered to be 

 a gelatinous secretion of the central capsule, and perhaps also the capsule-membrane, 

 in so far as it represents only a secondary excretory product of the unicellular organism. 



211. Adaptation. The innumerable and very various adaptive phenomena which 

 we meet with in the morphology of the Radiolaria, and especially in that of their 

 skeleton, are like other phenomena of the same kind, to be ultimately referred to altered 

 nutritional relations. These may be caused directly either by the influence of external 

 conditions of existence (nutrition, light, temperature, &c.), or by the proper activity of 

 the unicellular organism (use or disuse of its organs, &c.), or, finally, by the combined 

 action of both causes in the struggle for existence. In very many cases the cause to 

 which the origin of a particular form of Radiolaria is due may be directly perceived or 

 at least guessed at with considerable probability; thus, for example, the lattice-shells 



