114 THE AMERICAN ARBACIA 



Unfortunately the late larval characters of Echinus esculentus X E. acu- 

 tus, from which the Fg generation was obtained, are alike in the two 

 species, so that no information as to the inheritance could be obtained; 

 none of the Fg hybrids between E. esculentus or E. acutus X Ps. miliaris 

 which would have given the information, reached maturity.. Miss 

 Gordon from MacBride's laboratory raised some Arbacia plutei at 

 Woods Hole in 1926, using this method, but she was particularly 

 interested in the later development of the test, and gives no account 

 of the changes in the pluteus in her publication (1929). 



There are several varieties of Nitzschia closterium. There is a very 

 small form, Nitzschia closterium minutissima, the one used in the Plymouth 

 laboratory and now cultured at the Oceanographic Institution at 

 Woods Hole.' This is about 24 jj, long. A larger form from the New 

 Jersey coast has been cultured at Rutgers University; this measures 

 about 100 jj.. A still larger form grows in the Eel Pond at Woods Hole; 

 this measures about 200 [i. In the small variety, there are three types 

 of cells, the normal spindle-shaped cell, a triradiate cell and an oval 

 cell (D. P. Wilson, 1946). The relation of these three types to each 

 other and their division has been studied by Wilson. 



The method of feeding Echinoderm larvae with diatoms originated 

 with Caswell Grave (1902 a, b) at the Beaufort, N. C. laboratory, 

 where he was rearing the sand dollar Mellita testudinata. The diatoms 

 are swept into the mouth and oesophagus of the Arbacia pluteus by 

 means of very active cilia. The plutei thrive equally well on the small 

 and the large varieties oi Nitzschia closterium (E. B. H.). 



The very young sea urchins just after metamorphosis thrive better 

 on the calcareous protozoon Trichospherium, which furnishes the cal- 

 careous matter for the shell ; a little later they flourish on the red alga 

 Corallina, according to Shearer, de Morgan, and Fuchs (1914, p. 276), 

 for Echinus. These have not been tried with Arbacia punctulata. 



In some places the plutei do not require extra food for growth and 

 development, but apparently obtain sufficient food from the sea water. 

 This is the case with Arbacia punctulata at Beaufort, N. C. Brooks (1882), 

 and two of his students, Garman and Colton (1883) apparently raised 

 the plutei through metamorphosis without extra food. The sea water 

 there is rich in diatoms. 



' In a recent paper of N. I. Hendey (1954) entitled Note on the Plymouth "Nitzschia" Culture 

 in J. Marine Biol. Assoc'n, U. K. 33 : 335-339, the identification of the original culture of 

 Nitzschia of Allen and Nelson (1910) has been questioned, and identified by him as Phaeo- 

 dactylum tricornatum Bohlin, not a diatom but might be related to the Chrysophyceae. 



