1742 THE VOYAGE OF H.M.S. CHALLENGER. 



pear-shaped (Pis. 126-128). The thin siliceous wall of the galea has the same 

 irregular and delicate network as the valve from which it arises, and sometimes the 

 small irregular pores are also here reduced, so that the wall becomes partly solid. In 

 some cases the thin, solid, siliceous plate of the galea and of the valve is covered by an 

 irregular delicate network of crests ; the dimples between these crests may be easily 

 confounded with true pores. 



The cavity of the galea is filled with phseodella and does not communicate with the 

 cavity of the shell- valves, nor with the cavity of the radial tubes filled by jelly ; it is closed 

 towards the latter and the former by a thin solid plate of silex. Blitschli (1882, loc. cit., 

 p. 488) describes in Ccelothamnus a large circular opening (Taf. xxxi. figs. 2a, 4a), and 

 states that this is a direct communication between the cavities of the valves and of 

 their galeas which are called by him " der dreiseitige kastenformige Aufsatz " (e). This 

 error was caused by the fact that he observed the valves from the apical face only. 

 The apparent opening of communication does not exist, and is the optical section of the 

 rhinocanna, the shortened walls of which he describes as " trapezformige Kiesellamelle " 

 (7) ; the two lateral edges of the latter (" die seitlichen Zipfel," 8) are the paired frenula, 

 which connect the oi^en mouth of the rhinocanna with the base of the two frontal tubes. 



The "rhinocanna or nasal tube" (PI. 126, figs. 1, 4 ; PI. 127, figs. 4-9<) is a very 

 remarkable organ which is common to all Ccelographida (without any exception), and 

 distinguishes them markedly from all the other Radiolaria, and particularly from the 

 closely allied Coelodendrida, in which we find no trace of it. The rhinocanna is a 

 cylindrical or three-sided prismatic hollow tube, which l^^es in the sagittal plane, on the 

 outer surface of each valve, arises from the base of the galea, and is directed towards the 

 proboscis of the central capsule. The two opposite rhinocanuse open on each side of the 

 latter (PI. 127, figs. 4-9to), and usually this "nasal mouth" or the anterior opening of 

 the nose is somewhat dilated or even funnel-shaped. The posterior opening of the nose 

 passes directly over into the base of the cavity of the galea. 



Usually the rhinocanna is densely filled up by dark phseodella, which enter by this 

 channel into the cavity of the galea (PI. 127, figs. 4, 5, 9). Sometimes the entire 

 phseodium is enclosed in the two galeae and their rhinocannas (figs. 5, 9), whilst at other 

 times a great part of the phseodium lies outside of their cavities, and surrounds the pro- 

 boscis of the mouth, or even the anterior half of the central capsule (fig. 4). The length of 

 the cylindrical rhinocanna is usually about equal to that of the galea, whilst the diameter 

 of the latter is from three to five times as great as that of the former. The structure 

 of the thin wall is the same in both. The fine reticulation (fig. 8) is jiroduced either by 

 true, very small and irregular pores, or by a fine network placed on the solid thin wall. 

 We may distinguish on each rhinocanna an outer or distal convex face, which is opjDosite 

 to the proximal concave face of the galea, and an inner concave or proximal face, which 

 rests immediately upon the convex outer face of the shell-valve ; a thin solid lamella of 



