REPOET ON THE RADIOLARIA. Ixiii 



(pp. 89—127); for supplementary observations see E. Hertwig (1879, L. N. 33, p. 117) and Biitschli 

 (1882, L. N. 41, pp. 437-445). 



B. The Astropodia, or free radiating pseudopodia, are in many Radiolaria more or less clearly 

 distinguishable from the eollopodia, which form the sarcoplegma within the calymma ; how far these 

 distinctions depend upon a permanent differentiation (especially in the Acanthaeia and Ph^eodaria) 

 needs further investigation. 



C. The sarcodc-Jlar/elhim (perhaps better termed axoflagdlum) was first described in my 

 Monograph (1862, p. 115) in the case of various Discoidea (Taf. xxviii. figs. 5, 8 ; Taf. xxx. fig. 1). 

 Hertwig has given a substantially similar account of the organ in some other Discoidea 

 (L. N. 33, p. 67, Taf. vi. figs. 10, 11); probably this peculiar structure is confined to the order 

 Discoidea among the Spumellaeia, but is widely distributed within its limits. The axoflagellum 

 is a thick cylindrical thread of sarcode, finely striated and pointed towards its free end. It always 

 lies in the equatorial plane of the discoidal body, and always unpaired in one of its axes ; in the 

 triradiate Discoidea it is in the axis of the unjiaired principal arm and opposite to it (PI. 43, 

 fig. 15). In the Ommatodiscida (p. 500, PL 48, figs. 8, 19, 20) the axoflagellum probably passes 

 out through the peculiar marginal ostium of the shell. Perhaps it is always connected with the 

 central nucleus by intracapsular axial fibres, and is to be regarded as a specially differentiated 

 bundle of pseudopodia (or axopodia ?). 



95a. Tlie Myxopodia and Axopodia. — The two forms of pseudopodia which we 

 distinguish as myxopodia and axopodia differ markedly from each other both 

 morphologically and physiologically. The 'myxopodia, or ordinary free pseudopodia, 

 which are found in large numbers in all Radiolaria, and constitute their most important 

 peripheral organs, are simple homogeneous exoplasmic threads, which arise from the 

 sarcodictyum or extracalymmar sarcode network, and radiate freely into the water ; here 

 they may branch and combine by anastomosis to form a changeable network, but they 

 never contain an axial thread. The axopodia, on the other hand, are differentiated 

 pseudopodia, which consist of a firm radial thread, and a soft covering of exoplasm; 

 they penetrate the whole calymma in a radial direction and project freely from its 

 surface, and generally (if not always) they are produced inwards to the middle of the 

 central capsule, perforating its membrane ; their proximal end is lost in a dark central 

 heap of granules. Such axopodia are at present known with certainty only in the 

 AcANTHAKiA, where they are widely, and perhaps universally, distributed. Their develop- 

 ment in this legion probably stands in direct causal relation to the peculiar structure of 

 the central capsule and the centrogenous formation of the skeleton. Since the radial 

 skeletal rods of the Acanthometra possess originally a thin coating of protoplasm, 

 it may be said that the centrogenous axopodia of this group became differentiated in 

 two ways, the firm axial threads of one section remaining very thin and covered by 

 protoplasm, whilst those of the other section became metamorjihosed into radial bars of 

 acanthin. This hypothesis acquires more probability from the regular distribution and 

 arrangement of the axopodia in the Acantharia ; they usually stand at fixed intervals 



