REPORT ON THE RADIOLARIA. 11 



Johannes Miiller.^ The most common representative of it, the cosmopolitan Tlmlassi- 

 colla nucleata, was first described by Huxley in 1851. But as early as 1834 another large 

 Radiolarian, ajjpertaining either to this or to a nearly allied family, had been described 

 by Meyen as Phjsematium atlanticum. A third genus was detected by me in 1859 at 

 Messina and figured under the name Tlialassolampe margarodes.^ A very accurate 

 histological description of these forms was given in 1876 by Richard Hertwig.' 

 The same author figured in his Organismus in 1879 a very interesting simpler 

 form under the name Tlialassolampe primordialis (Taf. iii. fig. 5). Some similar 

 forms had already been observed by me, and are here united with it to form the first 

 genus Actissa.* 



Actissa is of the highest general interest as the most simple and typical form of all 

 Radiolaria, and as the common ancestral form, from which all other forms of this large 

 class may be derived. Its unicellular body exhibits neither the extracapsular alveoli of 

 Thalassicolla, nor the intracajjsular alveoli of Tlialassolampe, and shows all essential 

 characters of the Radiolarian tj^ae in its most simple form (PI. 1, figs. 1 to Ic). 

 The simple ceU-body is composed of a spherical central capsule and a concentric, 

 spherical, enveloping caljrmma, both separated by a thin membrane which is perforated 

 by innumerable pores. The capsule includes the endoplasm and in the centre a simple 

 spherical nucleus with nucleolus ; at the time of propagation this latter becomes cleft 

 into numerous small nuclei, each of which, together with a small piece of the surround- 

 ing endoplasm, forms a flagellated zoospore (fig. Ic). The extracapsulum is formed 

 by the large, structureless, spherical calymma or concentric jeUy-veil enveloping the 

 capsule, and by the thin granular matrix or the layer of exoplasm which separates 

 the calymma from the membrane. From this matrix or maternal tissue arise innumerable 

 very long and thin pseudopodia, as simple radiating filaments, the proximal part of 

 which is included in the caljrmma, whilst the distal part floats freely in the sea-water 

 (PI. l,fig. 1). 



The other ThalassicoUida differ from their common ancestral form, Actissa, mainly 

 by the higher histological differentiation of the unicellular body. Whilst in Thalassi- 

 colla and Tlialassolampe the nucleus remains a single s^^here as in Actissa, it becomes 

 branched or covered with radial bhnd saccules in Thalassojjila and Thalassopliysa ; 

 also the intracapsular protoplasm develops here a great variety of peculiar different 

 corpuscles, as oil-globules, pigment-granules, concentric concretions, crystals, &c. But 

 the most striking peculiarity by which the other ThalassicoUida differ from Actissa 

 is the development of large vesicular alveoh, either within or without the capsule ; 

 the unicellular body reaches by this inflation the extraordinary size of 5 to 10 mm. 

 or more. 



1 Abhandl. d. h ATcad. d. Wiss. Berlin, 1858, p. 28. « Monogr. d. Radiol., 18G2, Taf. ii. p. 2.53. 



2 Histologie der Eadiolarien, pp. 43-73, Taf. iii.-v. * Sitzunysh. med.-nat. Gesdkch. Jena,'Eehiu.ary 16, 1883. 



