SPECIES OF ECHINOIDEA AND ARBACIA LIXULA 6l 



to investigators who work at the Stazione Zoologica in Naples and 

 some other marine stations in Europe. The eggs of the two species are 

 similar, but that of ^. lixula is a little larger (79 (jl diameter) than that 

 of A. punctulata (74 [x diameter), is more heavily pigmented, has a 

 smaller peri vitelline space (1-2 [x), and a thicker hyaline layer (3 fx on 

 fertilization) E. B. Harvey, 1933 a, 1934, 1938 a. It stratifies similarly 

 with centrifugal force and is broken into two halves with about the 

 same force (10,000 X g for three minutes), but with this force into al- 

 most equal halves, the upper half containing oil, clear layer, mitochon- 

 dria, a little yolk, and the nucleus, and the lower half containing yolk 

 and pigment (E. B. Harvey, 1933 a, 1938a). See also Mortensen's 

 Monograph H : 566. 



There are four other species o{ Arbacia besides A. punctulata and A. 

 lixula. These are: A. stellata, or incisa, A. spatuligera, A. crassispina, and 

 A. dufresni. These are listed in the Classification, with localities where 

 they occur and references to Mortensen's Monograph. 



