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



for all the remaining forms of the Monopylea ; the great difficulty in their phylogenetic 

 derivation lies in the facts that, on the one hand, any one of the three elements may alone 

 constitute the skeleton, and on the other hand, in the great majority of the legion, two or 

 three are united together (compare §§ 182-185). 



112. The Skeleton of the Phwodaria. — The skeleton of the Ph^odaeia or Cannopylea 

 is always extracapsular, usually consists of a silicate of carbon (more rarely of pure silica), 

 and in the majority of the legion is composed of hollow cylindrical tubes, whose siliceous 

 wall is very thin, and whose lumen is filled with gelatinous material (§ 127). The mani- 

 fold and remarkable skeletal forms occuri'ing in this legion are not monophyletic, since 

 they cannot be derived from a common stem-form ; they are, on the contrary, poly- 

 phyletic, various skeletonless PniEODARiA (Phoeodinida) have independently acquired 

 skeletons of diff'erent form and composition. The legion Phwodaria can be subdivided 

 into four orders, the skeletons of which present the following important distinctions : — 

 (1) The Phaeocystina possess only incomplete Beloid skeletons (§ 115), composed of 

 many separate pieces, sometimes tangentially (Cannorhaphida, PL 101), sometimes 

 radially arranged (Aulacanthida, Pis. 102-105). (2) The PhEeosphseria form 

 Sphseroid skeletons (§ 116), usually only a simple lattice-shell without special aperture 

 (Pis. 106-111); two concentric shells united by radial bars occur only in the Canno- 

 sphaerida (PL 112). (3) The Ph aeogr omi a are distinguished by the formation of a 

 simple Cyrtoid skeleton (§ 123) resembling that of the Monocyrtida ; the monothalamus 

 lattice-shell is usually ovoid or helmet-shaped, more rarely polyhedral or almost spherical ; 

 a vertical main axis can always be distinguished, at the basal pole of which is an aperture 

 usually armed with teeth or spines (Pis. 99, 100, 1 13-120). (4) The P h jb o c o n c h i a 

 are distinguished from all other Radiolaria by the possession of a bivalved shell Like that of 

 the Conchifera ; the two valves of this Conchoid skeleton must be distinguished as dorsal 

 and ventral, as in the Brachiopoda (Pis. 121-128). The fifteen families of Ph.^odaria 

 which are arranged in the four orders just mentioned, present such great differences 

 among themselves, that the skeleton must be regarded as probably polyphyletic even 

 within the limits of each order. 



113. Types of Skeletal Formation. — No less than twelve different principal forms 

 may be distinguished as morphological types of the formation of the skeleton in the 

 Radiolaria ; some of these are peculiar to a single legion or even to. a smaller group ; but 

 sometimes the same form occurs in several legions. Some types occur only in an isolated 

 manner, independently of the others, but most exist in various combinations w-ith other 

 types. Of the twelve described below the Conchoid and Cannoid occur only in the Phwo- 

 daria ; the Plectoid and Circoid only in the Nassellaria ; the Astroid only in the 

 Acantharia ; the remaining seven types are found in several legions in the same form 

 and hence are polyphyletic. 



