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



Legion II. A C A 1^ T H A R I A, 



vel Actipylea, vel Acanthometrea (Pis. 129-140). 



Acantharia, Haeckel, 1881. 

 Actipylea. Haeckel, 1882. 

 Acanthometrea, Hertwig, 1879. 

 Panacantha, Haeckel, 1878. 



Definition. — Radiolaria with simple membrane boumling the central capsule, which is 

 everywhere perforated by innumerable fine pores (disj^osed either equally or symmetri- 

 cally). Extracapsulum without phseodium. Skeleton centrogenous (its growth proceeding 

 from the centre), acanthinic (organic, not siliceous). Fundamental form originally 

 spherical. 



The legion Acantharia vel Actipylea, to the extent here defined, was constituted 

 by me, 1878, in my Protistenreich (21. 102) under the name "Panacantha." A more 

 accurate definition of this group was given in 1879 by Hertwig under the name 

 Acanthometrea. Both names were replaced by me, 1881, in my Prodromus 

 (pp. 421, 465) by the more convenient name Acantharia. This legion comprises all 

 those Radiolaria which were first described by Johannes MiiUer, 1858, as Acanthometrae, 

 and also an important part of his Ilaliomma. In my Monograph (1862, pp. 371-424) 

 T disposed them in three families, Acanthometrida, Diploconida, and Dorataspida. 



Although the number of genera and species in this legion is much increased by the 

 rich collection of the Challenger, we can divide all Acantharia into two difi"erent orders : 

 Acanthometra (without complete lattice-shell) and A c a n t h p h r a c t a (provided 

 with a complete lattice-shell). 



The Acantharia agree with the Spumellaria in the structure of the simple capsule- 

 membrane, which is perforated by numerous small jjores (but constantly devoid of the 

 large main opening, which the Nassellaria and Ph/EODARIa possess, being hence united 

 as " Merotrypasta"). AVe can therefore unite both former legions as " Holotry]:)asta " 

 (compare above, pp. 5, 6) ; but in many Acantharia (if not in all ?) the numerous small 

 pores of the capsule-membrane exhibit a certain peculiar arrangement not observed in 

 the Spumellaria ; therefore the latter can be regarded as true " Peripylea" in opjaosition 

 to the former as "Actipylea." 



The peculiar main character of all Actipylea or Acantharia is determined by the 

 chemical constitution of their skeleton, which is not silex, but a peculiar organic 

 substance, called by me in 1862 "acanthin" (Monogr. d. Radiol., jjp. 30, 32). In all 

 other Radiolaria the skeleton is composed of silex or of a silicate. But besides this 



