1602 THE VOYAGE OF H.M.S. CHALLENGER. ' 



threads are simply united or confluent at the nodal points, and are not connected by a radial 

 or stellate septal junction, as in the Aulosphserida. There are, therefore, neither astral 

 septa nor a nodal cavity in each nodal point. The delicate shape of the thin and fragile 

 threads separates the Sagosphserida from the closely allied Orosphserida, the thick bars 

 of which contain a central axial canal and exhibit a concentric structure. Another differ- 

 ence between these two similar families is indicated by the form of the meshes of the 

 network, which are constantly triangular in the Sagosphserida, but irregularly polygonal 

 or quadrangular in the Orosphserida. The general habit of these two families, however, 

 is very different, since the big and stout spheres of the Orosphserida are the coarsest and 

 rudest spherical shells of all Radiolaria, whilst the fragile and delicate spheres of the 

 Sagosphserida represent the finest and most tender in the whole class. 



The spherical lattice-shell of the Sagosphserida has a considerable size, its diameter 

 being usually between one and three millimetres, rarely less or more. Some species are 

 very common and widely distributed, usually accompanying the common Aulosphserida ; 

 very frequently the similar shells of the two are found interwoven. But in spite 

 of this frequency and visible size, the Sagosphserida have hitherto almost completely 

 escaped the attention of observers. The main cause of this strange fact may be their 

 extreme delicacy and fragibility, so that complete and intact shells occur very rarely, the 

 majorit)'' being more or less broken and incomplete. It seems that only two species of 

 Sagosphserida have been hitherto observed. 



The first form described is Sagmarium trigonizon, observed by me in 1859 living at 

 Messina, and figured in 1862 in my Monograph as Dictyosoma trigonizon (Taf. xxvi. figs. 

 4-6), but afterwards called Spongodictyum trigonizon (loc. cit., p. 459). I supposed at 

 that time (now twenty-five years ago), that this remarkable and in many respects distinct 

 form might belong to the Spongosphserida, and that an internal, triple, spherical lattice- 

 shell, found entangled in its spongy framework, might be its central " medullary shell." 

 But at present, having found many shells of different Radiolaria accidentally entangled 

 in the arachnoidal framework of various Sagosphserida, I think it much more probable, 

 that that " triple medullary shell," composed of three simple concentric lattice-spheres, 

 was really a species of Plegmosphcera or . Actinomma, accidentally entangled in the 

 arachnoidal spongy framework of Sagmarium. This is the more probable, as I had 

 observed very frequently at Messina, in 1859, fragments of that framework, but only 

 once the triple lattice-shell which I supposed to be the "triple medullary shell" of the 

 former. The peculiar structure of the loose framework, its very large triangular meshes 

 and thin arachnoidal bars, partly provided with cruciate verticils (loc. cit., Taf. xxvi. 

 figs. 4, 5) have been very frequently observed by me during the last ten years in various 

 Sagosphserida (PH^EODARIA), but never in any true Sphseroidea (SPUMELLARIA). 



The second species of Sagosphserida hitherto observed, is Sagoscena gracilis, 

 described and figured in 1879 by Richard Hertwig as Aidospliara gracilis (Organism. 



