RADIOLAKIA 



853 



individual zooids, are aggregated into masses in which the skeleton 

 is represented only by scattered spicules, as in Spkcerozown (fig. 652) 

 and Thalassicolla. These 'sea-jellies,' which so abound in the seas of 

 warm latitudes as to be among the commonest objects collected by 

 the tow-net, are small gelatinous rounded bodies, of very variable 

 size and shape, but usually either globular or discoidal. Externally 

 thev are invested by a layer of condensed sarcode, which sends forth 

 pseudopodial extensions that commonly stand out like rays, but 

 sometimes inosculate with each other so as to form a network. To- 

 wards the inner surface of this coat are scattered a great number of 

 oval bodies resembling cells having a tolerably distinct membraniform 

 wall and a conspicuous round central nucleus. Each of these bodies 

 appears to be without any direct connection with the rest, but it 

 serves as a centre round which a number of minute yellowish-green 

 vesicles are disposed. 

 Each of these groups is 

 protected by a silicious 

 skeleton, which some- 

 times consists of separate 

 spicules (as in fig. 652), 

 but which may be a thin 

 perforated sphere, like 

 that of certain Poly- 

 cystina, sometimes ex- 

 tending itself into radial 

 prolongations. The in- 

 ternal portion of each 

 mass is composed of an 

 aggregation of large 

 vesicle-like bodies im- 

 bedded in a softer sar- 

 codic substance. 1 



From the researches 

 made during the ' Chal- 

 lenger ' Expedition, it 

 appears that the Radiolaria are very widely diffused through the 

 waters of the ocean, some forms being more abundant in tropical 

 and others in temperate seas ; and that they live not only at or near 

 the surface, but also at considerable depths. Their silicious skeletons 

 accumulate in some localities (in which the calcareous remains of 

 Foraminifera are wanting) to such an extent as to form a ' radio- 

 larian ooze ; ' and it is obvious that the elevation of such a deposit 

 into dry land would form a bed of silicious sandstone resembling the 

 well-known Barbadoes rock, which is said to attain a thickness of 

 1,100 feet, or a similar rock of yet greater thickness in the Nicobar 



1 See Professor Huxley (to whom we owe our first knowledge of these forms) in Ann. 

 Nat. Hist. ser. ii. vol. viii. 1851, p. 433 ; also Professor Miiller, of Berlin, in Quart. Journ. 

 Microsc. Sci. vol. iv. 1856, p. 72, and in his treatise Ueber die TJialassicoUen, Poly- 

 cystinen, und Acanthometren des Mittelmeeres, the magnificent work of Professor 

 Ha,eckel, Die Radiolarieii, and the monograph by K. Brandt, published in the Fauna 

 und Flora des Golfes von Neapel, 1885, 'Die kolonrebildenden Kadiolarien 

 (Spheerozoeen) des Golfes von Neapel.' 



FIG. 652. Sphcerozoum ovodimare. 



