TWO ABNORMAL PLUTEI OF ECHINUS. 
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the hydrocoele or rudiment of the wa ter- vascular system of 
the adult. The persistent neck uniting it with the rest of the 
left anterior coelom becomes the stone-canal. From the 
hinder end of the right anterior coelom a solid cellular out- 
growth is formed, which becomes hollowed out into a vesicle 
and comes to lie at the right of the madreporic pore. This is 
the so-called “dorsal sac ” or “madreporic vesicle/’ 
which I regard as a rudimentary antitnere of the hydrocoele 
on the right side. I have shown that in Aster ina gib- 
bosa (4) and in Ophiothrix fragilis (6) it may occasion- 
ally take on the form of a hoop with live lobes, thus copying 
Text-fig. 1. 
Five diagrams showing the changes undergone by the ccelomic 
sacs of the Echinopluteus larva, st. c. Stone-canal, liy. Left 
hydrocoele. m.v. Madreporic vesicle or right hydrocoele. 
the embryonic form assumed in these genera by the left 
permanent hydrocoele. The changes undergone by the coelom 
in the normal larva of Echinus are shown in Text-fig. 1, 
reproduced from my paper on the subject referred to above 
( 5 ). 
Now, in the living larva and in whole mounts of larvae* 
preserved in strong formalin, it is perfectly easy to see this 
madreporic vesicle lying at the side of the madreporic pore; 
but in the abnormal larva which we are considering no trace 
of such a structure was to be seen. We conclude, therefore, 
that in this individual the madreporic vesicle has taken on the 
same form as the normal left hydrocoele, and is therefore 
represented by the hydrocoele to be seen on the right side 
