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



Brandt has given further accurate information regarding their occurrence, constitution, and physio- 

 logical significance (L. N. 39, ii. Art., p. 235, figs. 62-73). 



11. The Endoplasm of the Peripylea. — The intracapsular protoplasm of the 

 Spumellaria or Peripylea is usually distinguished by a more or less complete radial 

 arrangement, which does not occur in the same form in other Radiolaria ; it may be 

 regarded as characteristic of this legion, for it probably occurs in all the species at some 

 period of life or other, and stands in a direct causal relationship with the typical structure 

 of the capsule-membrane in all the " Peripylea " (see note A). For as this is commonly 

 perforated by very numerous pores distributed at equal intervals over the whole surface of 

 the capsule, and since a communication between the intra- and extracapsular sarcode takes 

 place through these, the radiate structure of the endoplasm may be readily explained as 

 due to the influence of radial currents which take place continuously or intermittently 

 in the endoplasm. This radiate structure is most obvious when the endoplasm 

 contains no secondary products or only an insignificant amount of these, and thus 

 appears colourless and almost homogeneous, or only finely granular. Under these 

 circumstances, an optical section of the central capsule usually reveals a distinct radial 

 striation; numerous narrow, straight, dark streaks alternating regularly with still 

 narrower clear ones ; the latter consist of homogeneous, the former of more or less 

 granular protoplasm (PI. 20, fig. lo). Often there may be distinguished in each 

 darker streak a single straight row of strongly refracting (fat ?) granules, sometimes 

 several such rows. Occasionally the whole endoplasm becomes divided up into a number 

 of large " radial wedges," club-shaped, conical or pyramidal masses of granular protoplasm, 

 separated by clear divisions of hyaline plasma {e.g., in Actissa radiata, p. 14, where in 

 the optical section of the central capsule, between the membrane and the nucleus, 

 twenty-five dark radial wedges of equal size were separated by thick clear partitions of 

 hyaline protoplasm). In the majority of the Spumellaria this radial striation is partially 

 or entirely concealed by the formation of pigment or of other products. Very often it is 

 only visible in the cortical layer, which lies immediately below the capsule-membrane 

 (PI. 1, figs. 1, 3). The remarkable " centripetal cones" which characterise the Thalassi- 

 coUid genus Physematium, and were formerly described as " centripetal cell-groups," are 

 probably a special development of these cortical radial wedges ; they are conical cortical 

 bodies, regularly distributed on the inner surface of the membrane of the central capsule, 

 and disposed with the apex turned towards the centre (see note B). More rarely than in 

 the cortical layer, a similar radial structure is to be found in the innermost medullary 

 layer immediately surrounding the nucleus. Here the endoplasm sometimes breaks up 

 into fine radial threads, which are anatomically separable and hang down from the free 

 nucleus as thin processes (see note C). In some cases it is also possible to isolate radial 

 rods from the cortical layer of teased out central capsules. 



