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



ment of the radial tubes in most Ph.eodaria is indefinite and very variable ; only in a few 

 families is the number constant in each species and genus, and the disposition regular. 

 The Medusettida (Pis. 118-120) resemble the Nassellaria, inasmuch as equal radial 

 feet diverge from the base of their shell, sometimes three in number {Cortinetta, PL 117, 

 fig. 9), sometimes four [Medusetta, PL 120, figs. 1-4), sometimes six (Gazelletta) ; 

 Gorgonetta is specially distinguished by the possession of six ascending and six descend- 

 ing spines regularly alternating (PL 119). The Tuscarorida (PL 100) usually have 

 three or four equidistant feet. The Circoporida (Pis. 115-117), on the other hand, 

 rather approach the S p h se r o i d e a, their spherical or regular polyhedral shell having 

 a definite number of tubular radial spines, which arise at regular intervals from their 

 angles ; Circoporus has six, Circospathis nine, Circogonia twelve, and Circorrhegma twenty 

 radial tubes. Very rarely the tubes of the Ph.eodaria are angular, usually they are 

 round, more or less cylindrical, though they are often bifurcated or even ramified, and 

 exhibit a great wealth of the most delicate appendages ; siliceous hairs, bristles, spines, 

 barbed or anchor-like hooks, spathillte, brushes, circlets, &c. (compare Pis. 99-128). 



140. Main-Spines and Accessory Spines. — As accessory spines (Paracanthse) we 

 oppose to the main-spines ( Pro tacan thee), just described, all those processes which have 

 no determining influence upon the formation of the skeleton as a whole, but are to be 

 regarded as secondary constituents of the skeleton, or appendicular organs of inferior 

 significance. They are developed in the utmost variety, sometimes as hairs or bristles, 

 sometimes as thorns or clubs, either straight or curved (often zigzag), smooth or 

 barbed; sometimes standing vertically upon the shell, or directed towards the centre, 

 sometimes obliquely, or rising at a definite angle. In those Spumellaria whose lattice- 

 sheU consists of several concentric spheres, the accessory spines generally arise from the 

 outermost, the main-spines, on the contrary, from the innermost. In the Nassellaria, 

 multifarious forms of accessory spines are especially developed in the order P 1 e c t e 1- 

 laria. In the Ph^odaria they are often furnished with delicate appendages, e.g., 

 anchor-hooks, spathiUse, coronets, &c. Among the Acantharia the accessory spines 

 which arise from the surface of the shell in the Acanthophracta are very charac- 

 teristic. They are not radially disposed (like the similar superficial sj^ines of the 

 Spumellaria), but parallel to the radial main-spines from whose transverse processes they 

 arise. Since in aU these Acanthophracta the twenty radial main-spines are opposite 

 to each other in pairs, all the accessory spines (often several hundred) are parallel to ten 

 difierent regularly disposed axes of the lattice-shell (Pis. 134r-138). 



The skeletons of the Eadiolaria, in addition to the general relations which have been discussed 

 above, present numerous and important special differences in the various larger and smaller groups. 

 These are indicated in detail in the descriptions of the legions, orders, and families in the systematic 

 portion of this Eeport. 



