Ixvi THE VOYAGE OF H.M.S. CHALLENGER. 



Radiolaria, and appears to undergo more significant differentiations than that of the 

 three other legions. Since the pores in the wall of the central capsule are not distri- 

 buted evenly and at equal intervals over its whole surface (as in the PERIPYLEA), but 

 rather exhibit a regular disposition in groups at unequal intervals, the number of pro- 

 jecting pseudopodia is much less and the law of their arrangement different from that 

 which obtains in the PERIPYLEA ( 58). In many and probably in all ACANTHARIA they 

 are divided into two groups, those which arise from the centre of the capsule and possess 

 firm axial threads, and those which have not these characters (compare 95, A). The 

 axopodia, or stiff pseudopodia with axial threads, arise from the centre of the capsule, 

 are present in much smaller numbers than the soft and flexible myxopodia, and are 

 regularly disposed between the radial bars of acanthin, usually so that they are as far 

 removed from them as possible, i.e., in the centre between each three or four bars ; 

 these latter may indeed be regarded as strongly developed axial threads, which have 

 become changed into acanthin (95, A). The soft myxopodia, or pseudopodia without 

 axial threads, are much more numerous than the others, and arise from the sarco- 

 dictyum or exoplasmic network which ramifies over the surface of the calymma. 

 Their number and arrangement seem, however, in many (if not in all) ACANTHARIA to 

 be regular and not to possess the extraordinary variability seen in the other three 

 legions. In many Acanthometra the sarcodictyum exhibits a symmetrical con- 

 formation, with regular or subregular, polygonal (mostly hexagonal) meshes, and 

 generally the stronger threads of the sarcodictyum secrete a firm, homogeneous or 

 fibrillar, striated substance, which forms a network of ridges on the surface of the 

 calymma. In the Acanthophracta the place of this is taken by the acanthin 

 network of the primary lattice-shell. The axopodia of the Acanthometra are 

 usually about as long as the radial spines between which they stand ; their stiff axial 

 thread is surrounded by a soft sheath of protoplasm, communicating with -the thin 

 sarcomatrix which surrounds the central capsule. Numerous branches pass into the 

 calymma from the exoplasmic sheath of the axial threads, and form by their interweaving 

 a loose sarcoplegma. The most peculiar differentiated products of the exoplasm of the 

 ACANTHARIA, however, are the myophane fibrillse of the Acanthometra, which have 

 already been described under the name of myophriscs ( 96). 



99. The Exoplasm of the Monopylea. The extracapsular protoplasm of the NASSEL- 

 LARIA or MONOPYLEA arises only from the porochora, or the intracapsular podoconus, 

 the oral base of which is formed by this porous area. The pseudopodia or protoplasmic 

 threads which pass through the pores of the latter, united into a bundle, are not very 

 numerous (in most NASSELLARIA probably between thirty and ninety), and unite just 

 outside it to form a thick discoid sarcomatrix ; this covers the porochora completely 

 below, and spreads out in the form of a thin envelope of exoplasm over the whole 



