82 HENRY BURY. 
the water-vascular ring in the same adradius as the water- 
pores. 
Biitschli (6) has already called attention to the fact that in 
Lovén’s figure of a young Echinid (11, pl. xxi, fig. 170) the 
flattened side of the dorso-central plate is directed towards 
interradius E (compare fig. 36), and he infers from this that 
the anus was originally in this position. I had observed the 
same point in 18 88, before Butschli’s paper appeared, and am 
able to give proofs of that which he could only conjecture. 
As the anus does not break through till the young Echinus 
microtuberculatus has attained a diameter of from 5 to 7 
mm., it is no difficult matter to obtain the necessary material; 
and by adding a few drops of chloral hydrate to the water 
containing the animals, they can be stupefied in an extended 
condition and easily examined. 
Fig. 37 shows the position of the anus in one of the 
youngest specimens in which it could be detected with absolute 
certainty; it will be seen that it lies well within interradius 
E. As the small plates of the perianal area (which sometimes 
show a marked bilateral symmetry) increase in number, the 
anus works gradually across into radius IV (fig. 38), and 
thence into its adult position; but it is quite unnecessary to 
illustrate the whole series of changes. 
I have not succeeded in tracing the steps by which the long 
and tortuous alimentary canal of the adult is evolved out of 
the globular stomach and short intestine of the Pluteus. The 
anus closes at the moment metamorphosis begins, and in my 
youngest radial specimens I cannot even trace the intestine 
with certainty. It is, however, interesting to note that in the 
adult the cesophagus lies in interradius A, and from this 
point the alimentary canal makes a complete circuit of the disc 
till it again reaches this interradius. Then it turns back 
(compare 14, pl. xiii, fig. 7) as far as interradius B, from which 
it runs sharply back again to the anus in interradius D. 
I am almost certain that in the youngest specimen of which 
I have sections (diameter 1 mm.) this last loop (from inter- 
radius D to interradius E and back again) does not exist, 
