BLASTULA, GASTRULA AND PLUTEUS II3 



c. Development of Pluteus When Not Fed 



At Woods Hole, the pluteus reaches its maximum size in three to four 

 days, and will not continue to grow unless it is specially fed. At its 

 maximum (without feeding), the long anal arms measure about 400 [x; 

 the longest ones in my cultures were 442 [x from base to tip. Cultures 

 of these plutei may be kept in the laboratory, if the sea water is changed 

 every day or so, for three or four weeks. The plutei gradually get smal- 

 ler by resorption of the arms, and the body takes on a bloated appear- 

 ance (Plate XVI, Photograph 10). There is apparently sufficient food 

 material in the sea water for them for three of four days after hatching, 

 and there is considerable growth, but after this they degenerate unless 

 supplied with additional food. It seems to make no difference in their 

 growth if supplied with extra food before the fourth day. 



d. Food for Pluteus 



The best food for sea urchin larvae has been found to be the diatom 

 JVitzschia closterium, but they will grow on other diatoms, e.g., Lich- 

 mophora (E. B. H.). The Mitzschias themselves must be raised in pure 

 culture and require a special diet, Miquel's solution ^ The method has 

 been worked out by Allen and Nelson (1910) in Plymouth, England, 

 and has been used by many investigators at the Plymouth laboratory. 

 Shearer, de Morgan, and Fuchs (1914) have in this way succeeded not 

 only in raising the normal plutei of several species of sea urchin to 

 maturity, but have also raised some hybrid plutei to maturity. Fuchs 

 (1914b) has even obtained the next or F2 generation of these hybrids. 



' Miquel's solution as modified by Allen and Nelson (1910), consists of: 



To each liter of sea water add 2 cc. Solution A andi cc. Solution B, and sterilize by heating 

 to 70° C. When cool, decant off the clear liquid from the precipitate, which will have formed 

 when Solution B is added to the sea water. 



Ketchum and Redfield (1938) have used a slight modification. 



A very simple medium for growing Nitzschia has been used by John Ryther at the Woods 

 Hole Oceanographic Institute (personal communication, Sept. 1954): Add to sea water 

 at 20° C. or below 



Na2HP04i2 HjjO .002% 

 KNO3 .01 0/0 



