A. EARLAND ON RADIOLARIA. 259 



interior of the capsule, and may be cleiiignated the " thread cone " 

 (jjodocomis). See Figure 3a. 



In the fourth legion, the Oannopylea or Phceodaria, the capsule 

 membrane is double, and possesses only a single large main open- 

 ing or osculum, which is situated at the basal pole of the vertical 

 main axis and is closed by a circular radiated lid {operculum 

 radiatum). This operculum appears when examined from the 

 surface as a sharply defined stellar area (astropyle) from the 

 middle of which arises a shorter or longer cylindrical tube, the 

 proboscis. Most Phseodaria also possess several (usually two) 

 smaller accessory openings or '' parapylae," generally near the 

 aboral pole of the main axis. See Figure 4a. 



From the foregoing definitions it will be seen that the four 

 legions fall naturally into two pau'S. The first and second legions, 

 the Peripylea and the Actipylea, agree in having the capsule 

 membrane pierced with numerous fine pores, but have no principal 

 opening or osculum. The third and fourth legions, the INIono- 

 pylea and Oannopylea, have as a rule but a single osculum, at 

 the basal pole of the vertical or main axis. The first and second 

 legions, therefore, are formed into a separate group, the Porulosa 

 or Holotrypasta, and the third and fourth into another group, 

 the Osculosa or Merotrypasta. 



The Central Capsule and its Contents. 



The central capsule of the Radiolarian cell is, in its origin, no 

 doubt, a geometrical sphere, with all parts concentrically and evenly 

 arranged round its centre, the nucleus. This primary spherical 

 form, however, becomes modified in most Radiolaria into various 

 secondary forms, which are chiefly dependent on the formation of 

 the skeleton. The central capsule, increasing in size after the 

 formation of the hard skeleton has begun, has of course to adapt 

 itself to the shape and growth of that shell. Passing through 

 the interstices in the skeleton, it frequently reunites outside it, so 

 that parts of the skeleton become enclosed in the capsule, although, 

 except in one legion, the Acantharia, the skeleton is always 

 originally formed outside the central capsule. 



Normally the central capsule consists of three parts : (1) The 

 capsule membrane, the nature of which has already been described ; 



