306 
PKOF. E. W. MACBKIDE. 
of other Spatangoidea, is the possession of an aboral 
process of the body supported bya special skeleton. 
This, according to Vernon, is only developed on the fifth day, 
but according to him it appears in all the hybrid larvae, 
although in tliem it is shorter than in the normal larva. 
Tennent ( 21 ), in a short preliminary notice read before the 
International Congress of Zoology held in Boston in 1907, 
had already announced that he had crossed the eggs of 
Moira, a Spatangid, with the sperm of Strongylocen- 
trotus. He gives no description of the hybrids, however, 
but in a later paper gives more details ( 23 ). He succeeded in 
fertilising the eggs of Moira with the sperm of Toxo- 
pneustes. He kept the (larvae alive for seven days. None 
of them developed the aboral spine. Nevertheless, he says 
that they were of the “ maternal intermediate type.” He also 
made the reciprocal cross and obtained similar “inter- 
mediate ” larvae ! Some were of the purely maternal type 
and developed for twenty days, but the possibility is not 
excluded that these were developed from eggs fertilised by 
chance spermatozoa of their owu species. 
Since the results which 1 obtained differ markedly from those 
recorded by Vernon, and are in many respects different from 
those obtained by Tennent, it may be worth while to give some 
account of the methods employed. At Millport large num- 
bers of both E c h i n u s e s c u 1 e n t u s and of E c h i n o c a r d i u m 
cordatum were tivailable. The former species could be 
picked off the rocks^a stone's throw from the laboratory at 
ordinary low tides ; the latter species could be obtained by 
the bucket-full at low spring tides by digging in a sandy 
beach about half a mile from the laboratory. The tank-water 
in the laboratory was unpolluted by drainage of any kind, 
for the laboratory is situated over a mile and a half from the 
centre of the little town'of Millport, and the full tidal current 
of the Firth of Clyde sweeps past the rocks on which it is 
built. In the water [in the tanks the adults of both species 
live comfortably, but it is, nevertheless, inimical to the 
larVcB. The reason of this is to be found in the metallic 
