564 DEPTHS OF THE OCEAN 



cruise of the " Challenger " Sir John Murray captured them from a boat 

 in calm weather floating at the surface of the ocean, where they were 

 just visible to the naked eye. On the ocean-floor in moderate depths in 

 tropical and sub-tropical regions the dead shells occur in such enormous 

 numbers that the deposit is called Globigerina ooze. The species and 

 individuals decrease in number as we go north or south from the tropics, 

 and in the Norwegian Sea only one species, viz. Globigerina biilloides 

 (see Fig. 118, p. 150), occurs in any abundance either at the surface or in 

 the bottom deposits.^ 



Radiolaria. The Radiolaria occur in a profusion of species. The cell possesses a 

 central capsule containing the nucleus or nuclei and an outer layer of 

 protoplasm capable of throwing out very thin threads (pseudopodia). 

 The skeleton is developed in various ways and facilitates the dis- 

 crimination of an enormous number of sharply separated forms (see Figs, 

 no to 117 in Chapter IV.). In his report on the "Challenger" Radio- 

 laria, Haeckel described no less than 20 orders, 85 families, 739 genera, 

 and 4318 species, taken partly from the deposits and partly in the tow- 

 nettings ; in one single bottom sample from 4475 fathoms in the Pacific 

 338 species were found. The Radiolaria are wholly pelagic, and occur 

 in all oceans where the salinity is not too low (as it is in the Baltic), 

 over deep water as well as over shallow water, attaining their maximum 

 development in the Pacific. 



In order to discuss their distribution we may mention some of the 

 typical groups :■ — ■ 



The Acantharia are mostly spherical ; the perforations of the central 

 capsule are regular. The skeleton consists of acanthin, a peculiar elastic 

 organic substance, in the form of twenty needles radiating from the 

 centre of the sphere. The majority of the species occur in tropical 

 waters and in the upper layers of the ocean. They are divided into two 

 groups, Acanthometra and Acanthophracta. 



In a vertical haul in the Atlantic Popofsky- found no less than 

 75 species of Acanthometra alone, and a haul in the Indian Ocean 

 procured a similar number. North and south from the equator the 

 number of species decreases, the majority living between lat. 40° N. and 

 40° S. The different regions of this warm belt have many species in 

 common. According to Popofsky the total number of known species 

 is 179, of which only 18 have been found in the Atlantic to the north of 

 lat. 50° N., and 10 of these are known only as casual or seasonal visitors. 

 The commonest forms in northern waters are Acanthochiasmafusiforme, 

 Acanthometron pellucidiim (Fig. 389), AcantJionidium ecJiinoides (Fig. 390), 

 Phyllostaurus quadrifolius, Acanthostaurns nordgaardi {¥\g. 391). 



It is generally supposed that the temperature limits the bathy- 

 metrical distribution of the Acantharia, just as it is known to limit their 

 horizontal occurrence. In the Atlantic the German Plankton Expedition 

 found the deepest living species at a temperature of 9.4° C. In the 

 Mediterranean, where high temperatures occur deeper, they have been 



^ See Murray, "On the Distribution of the Pelagic Foraminifera at the Surface and on the 

 Floor of the Ocean," Nattiral Science, vol. xi. p. 17, 1897. 



"^ Popofsky, " Acanthometriden," Ergeb. Plankton-Expedition, Bd. iii. , 1904; "Die nor- 

 dischen Acantharien," Nordisches Plankton, No. xvi. 



