REPORT ON THE RADIOLARIA. 11 



Johannes Mliller. 1 The most common representative of it, the cosmopolitan Thalassi- 

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

 Eadiolarian, appertaining either to this or to a nearly allied family, had been described 

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

 Messina and figured under the name Thalassolampe margarodes. 3 A very accurate 

 histological description of these forms was given in 1876 by Richard Hertwig. 3 

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

 form under the name Thalassolampe 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 

 Tfialassicolla, nor the intracapsular alveoli of Thalassolampe, and shows all essential 

 characters of the Radiolarian type in its most simple form (PL 1, figs. 1 to Ic). 

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

 spherical, enveloping calymma, 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 jelly-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 calymma, whilst the distal part floats freely in the se*water 



(PL 1, % I)- 



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

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

 colla and Thalassolampe the nucleus remains a single sphere as in Actissa, it becomes 

 branched or covered with radial blind saccules in Thalassopila and Thalassophysa ; 

 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 Thalassicollida differ from Actissa 

 is the development of large vesicular alveoli, 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. Ic. ATcad. d. Wm. Berlin, 1858, p. 28. 2 Monogr. d. Radiol., 1862, Taf. ii. p. 253. 



3 Histologie der Radiolarien, pp. 43-73, Taf. iii.-v. 4 Sitzungsb. med.-nat. Geselhch. Jena, February 16, 1883. 



