332 DEPTHS OF THE OCEAN 



wreath. Calciosolenia 7nurrayi resembles, to some extent, the 

 shape and structure of Rhizosolenia, as the shields of lime are 

 not rounded like those of most other species, but rhomboid and 

 spirally bent, so that between them they form a cylindrical tube, 

 pointed at either end, and furnished at the extremities with one 

 or two fine calcareous setae. 



Notwithstanding their small dimensions these microscopic 



calcareous algae oc- 

 C\^ \ I cupy a very important 

 place in the economy 

 of the sea, and their 

 shields of lime, which 

 may be met with in 

 geological deposits 

 dating from as far 

 back as the Cambrian 

 period, show that they 

 have retained their 

 shape practically un- 

 altered through im- 

 measurable ages. 

 They are almost en- 

 tirely oceanic, and 

 mostly belong to the 

 warmer seas. In 

 coastal waters, where 

 the salinity is lower, 

 they are scarcer, but 

 the commone st 

 species, the little 

 Pontosphcera ktixleyi, 

 has been found even 

 in the Baltic, and 

 there were such immense quantities of it in the inner parts of 

 the Christiania fjord during the hot summer of 191 1 (5 to 6 

 million cells per litre) that the calcareous cells with their strong 

 refraction gave the sea quite a milky appearance. 



The naked flagellates in the sea are still only imperfectly 

 known, though, no doubt, the part they play is quite a consider- 

 able one. In coastal waters they occur sometimes in such 

 abundance that we have actually been able, even with our present 

 defective methods, to discover and describe a number of species. 

 In the open sea we are best acquainted with the passive and 



Fig. 239. 



-Different Types of Coccolithophorid.^i. 



Mickaeharsia elegans ; 2, Ophiaster formosi/s ; 3, Rhabdo 

 spheera claviger ; 4, Syracosphcsra frolongata ; 5 

 leriia murrayi ; 6, 7, Coccolithophora leptopora ; 

 sphcera ktixleyi. 



Calcioso- 

 , Portio- 



