6o THE AMERICAN ARBACIA 



yolk, into a large nucleate half of 152 y. diameter and a very small 

 lower half of 80 y. diameter. When fertilized, the lower non-nucleate 

 half containing a little yolk, clear layer, and mitochondria, develops 

 much better than the upper half containing oil and yolk and the nucleus. 



Further information concerning the distribution, morphology, and 

 larval development may be found in Mortensen's Monograph, 1943, 

 vol. Ill, Part. 3, p. 198. 



The sand dollar, Echinarachnius parma, is abundant at Woods Hole 

 and breeds during the summer, so that it is excellent material for ex- 

 perimental work at the Marine Biological Laboratory. The eggs are 

 best early in the season, June and July. The egg is large, 145 [jt. in dia- 

 meter, and is unpigmented. It is surrounded by a jelly coat 95 ]x thick 

 or less, in which are imbedded large red pigment granules 6-8 \x in 

 diameter. After fertilization there is a large perivitelline space 30 [x 

 across; the hyaline layer is so thin as to be unmeasureable at any time. 

 Development is a little slower than in Arbacia; first cleavage occurs 

 at 23° C. in i^ hours [Arbacia 50 min.), and they swim in 16 hours 

 [Arbacia 8 hours). Echinarachnius parma from Maine is a larger animal, 

 averaging 8 cm. in diameter while the one at Woods Hole averages 

 5 cm., but the egg measurements are the same. With centrifugal force, 

 the mature unfertilized egg stratifies with yolk granules above, then 

 the clear layer, then granules consisting of two kinds, the upper, finer 

 granules being mitochondria, staining with methyl green and Janus 

 green, and the bottom layer of coarser granules and staining with neutral 

 red and methylene blue. Usually there is no oil cap, but sometimes there 

 are a few oil drops at the centripetal pole; the nucleus lies as usual at 

 the centripetal pole. There is sometimes a clear layer here as well as 

 below, as in photograph 14 of Costello (1939). With sufficient force, 

 10,000 X g for twelve minutes, the egg breaks across the clear layer 

 into a large upper half (126 fx diameter) and a smaller lower half 

 (100 [i. diameter). Both parts develop after fertilization. See Costello, 



1932, 1939- 



Two other species oi Echinoidea were dredged by fishermen in August 

 1954, about 600 ft. down, 90 miles south east of No Man's Land. One 

 was a large dull grey urchin with heavy spines, tentatively identified 

 as Cidaris abyssicola A. Agassiz. The other was a rather beautiful smaller 

 urchin, reddish with long spines, tentatively identified as Coelopleurus 

 Jloridanus A. Agassiz. These were quite new to the Woods Hole region. 

 Both are described by Mortensen, the first in his Monograph I : 301, and 

 the second in II : 612. 



There is another species of Arbacia, A. lixula (pustulosa), accessible 



