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E. AY. MACBRIDE. 
oesophagus and mouth (m.) can be made out. All the 
structures which I have just described can be seen in a normal 
larva of this age when viewed from the left side, but the 
peculiarity of the larva under discussion is this : that when, as 
is represented in fig. 4, it is viewed from the right side, an 
almost precisely similar set of structures can be seen. It 
is, of course, not quite easy to get the larva to lie flat 
on either side, and as a matter of fact in fig. 3 it is 
tilted so that the ventral surface of the oral lobe is half 
turned towards the observer, and we see into the larval mouth 
( st .), whereas in fig. 4 the dorsal surface of the oral lobe is 
presented to our view, and the larval oesophagus is seen 
glimmering through it. The same arrangement of spines and 
tube-feet can be seen on the right side as on the left, and 
within them a circle representing a second nerve-ring; within 
this a pentagon representing a second but rather smaller 
Aristotle’s lantern ; and finally within this a second adult 
oesophagus and mouth. The only differences between the left 
and the right discs which I could detect was the smaller 
radius of the nerve-ring, and the smaller size of the contained 
Aristotle’s lantern on the right side. 
Now we have already concluded from the study of the 
abnormal larva of Echinus miliaris that whereas the 
appearance of a second hydrocoele is probably a reminiscence 
of a state of affairs which obtained in the free-swimming 
ancestor of all Echinoderms, the appearance of a second 
amniotic cavity could not be interpreted in that sense. It is 
still clearer that the appearance of the peculiar spines of the 
adult Echinoid on the right side cannot be an atavistic 
feature, and still less the appearance of a second Aristotle’s 
lantern and second “ adult oesophagus.” 
In order to show what relationship to each other the 
various structures sustain which together make up the 
complex known as the “ echinus-rudiment,” I reproduce 
here a text-figure from my paper (5) on the development 
of echinus esculentus. 
From these diagrams we see that the amniotic cavity 
