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



are to be seen in some PhEeosphaeria {Oroj^legma, PI. 107, fig. 1; Sagoplegma, 

 PI. 108, fig. 2; Auloplegma, PI. Ill, fig. 8). No Spongoid skeletons are known 

 among the Acantharia. 



127. The Cannoid Sheleton. — Cannoid or tubukxr skeletons are those which are com- 

 posed of hollow tubes; they occur exclusively in the Ph.'EODAria or Cannopylea. 

 Tubular processes, nevertheless, occur in some other Radiolaria, as, for example, among 

 the Spumellabia in a portion of the CoUosphserida (SiphonosphcBra, CaminosphcBr-a, 

 Pis. 6, 7), and of the Prunoidea {Pipetia, Cannartus, &c., Pk 39, figs. 6-10, &c.), 

 also among the Nassellaria in Theosyringium (PI. 68, figs. 4-6), Cannohotrys (PI. 96, 

 figs. 3, 4, 8-11, 20-22), &c. In all these cases, however, the tubes are direct processes 

 of the cavity of the shell, the trabeculas of the lattice-work being solid. Only in the 

 Cannopylea are the lattice-bars themselves, the radial spines and appendicular organs, 

 generally tubular (hence the designation " Pansolenia "). The lumen of the thin-waUed 

 siliceous tubes is filled with jelly, and hence the specific gravit}' of the relatively large 

 skeleton is considerably diminished. This peculiarity is not found in all Cannopy'Lea ; it 

 is wanting in all Sagospharida and Concharida, as well as in a part of the Orosphserida 

 and Castanellida ; in the latter there are found intermediate stages between hollow and 

 solid skeletal rods. Very often a fine siliceous thread runs in the axis of the tubes, which 

 is connected with its wall by lateral branches (Pk 110, figs. 4, 6 ; PI. 115, figs. 6, 7). 

 More seldom the tubes are divided by horizontal septa into a series of chambers (Meduset- 

 tida. Pis. 118-120). The two famUies Aulosphaerida (Pis. 109-111) and Canno- 

 sphserida (PL 112) are distinguished from all other PHxEODaria by the fact that their 

 tubes are separated by astral septa in the nodal points of the lattice-shell (§§ 112, 134). 



128. The Conchoid Skeleton. — By the name " Conchoid skeletons" are distinguished 

 the bivalved lattice-sheUs which occur exclusively in the legion Ph^odaria ; they are 

 quite characteristic of the Phseoconchia or PhcBodaria hivalvia, which embrace 

 three families :— Concharida (Pis. 123-125), Coelodendrida (Pis. 121,122), and 

 Coelographida (Pis. 126-128). The two valves of the lattice-shell of the Concharida 

 are simple, hemispherical, or boat-shaped, whilst in the Coelodendrida and Ccelographida 

 tubes grow out from them, which branch and usually give rise by anastomosis to a second 

 external bivalved shell. In all P h ae o c o n c h i a the two valves are so disposed about 

 the central capsule that an open slit remains between them, into which open the apertures 

 of the central capsule; and since all these PhcBodaria conchoidea are Tripylea, with three 

 typical openings in the central capsule, and since the two lateral accessory openings lie 

 at either side of the aboral pole, and the unpaired main-opening at the oral pole of the 

 main axis, it follows that the two valves are to be regarded as dorsal and ventral as in 

 the Brachiopoda (not right and left as in the Lamellibranchiata). The dorsal and ventral 



