34 



Marine Invertebrates 



By using Miquel's seawater Allen and Nelson (1910) were able to 

 obtain persistent cultures of the following species of diatoms: Asterio- 

 nella japonica, Biddulphia mobiliensis, B. regia, Chaetoceras densum, 

 C. decipiens, C. constrictum, Cocconeis scutellum, Coscinodiscus excen- 

 tricus, C. granii, DityUum brightwellii, Lauderia borealis, Nitzschia 



— -. closterium, N. closte- 



rium forma minutissima, 

 N . seriata, Rhizosolenia 

 stoltcrjothii, Skeleto- 

 nema costatum, Strep- 





totheca thamensis, and 

 Thalassiosira decipiens. 

 Of all these species 

 the cultivation of Nitz- 

 schia closterium /. minu- 

 tissima is more widely 

 practised in many lab- 

 oratories both in the 

 United States and in 

 Europe than that of any 

 other diatom. This is 

 primarily due to its 

 small size and its ability 

 to remain in suspension 

 for a long period of 

 time, these qualities 

 rendering it very useful 

 as food for small plank- 

 tonic organisms. 



Light and temperature 

 are the two principal 

 physical factors which 

 govern the rate of prop- 

 agation of diatoms. The 

 growth of cultures left in the laboratory and subject to rather wide fluc- 

 tuations in temperature and intensity of illumination are greatly affected 

 by these changes. Temperatures above 22 ° C. are obviously harmful to 

 a Nitzschia culture and when the summer heat approaches 30 , as hap- 

 pens regularly in the author's laboratory of the U. S. Bureau of Fisheries, 

 they may perish. To avoid this difficulty and to keep the cultures in 

 health, both temperature and illumination should be kept constant. 



The following easily constructed arrangement has been successfully 

 used by the author. Culture flasks are kept in a cabinet (Fig. 34), 60" 



Fig. 34. — The cabinet for diatom cultures. A, air 

 switch; C, dry cells; E, relay; F, fan; H, small 

 heater; M, metastatic temperature controller; P. 

 pipes to refrigeration plant; R, refrigeration unit; 

 S, wall switch; T, transformer. 



