TWO ABNORMAL PLUTET OF ECHINUS. 
235 
Two abnormal Plutei of Echinus, and the light 
which they throw on the Factors in the 
normal development of Echinus. 
By 
Prof. E. W. UlacBiide, D.Sc., LL.D., F.R.S., 
Chief Assistant in the Department of Zoology, Imperial College of 
Science and Technology, London. 
With Plate 24 and 2 Text-figures. 
The present paper deals with, two remarkable larvae, the 
first a larva of Echinus mi liar is obtained in a culture 
reared from eggs fertilised in the Zoological Laboratory of 
the Imperial College of Science and Technology, and the 
second a larva of Echinus esculentus obtained in a culture 
of eggs fertilised in the Laboratory of the Marine Biological 
Station at Plymouth. The second and more interesting 
specimen I owe to the kindness of Mr. de Morgan, by whom 
the culture in which it appeared was instituted. 
I shall first deal with the larva of Echinus miliar is. 
The sea-urchin from which the eggs were obtained was sent 
alive from Plymouth to London. Here the eggs were 
fertilised with sperm from another individual, which also 
came from Plymouth, and the resulting larvae were transferred 
to half-gallon jars of green bottle-glass containing sterilised 
sea-water to which a culture of the diatom Nitschia had been 
added. The jars were placed in a sink into which water was 
constantly dropping from a tap and they were covered in. 
Here the larvae developed for some weeks until a six-armed 
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