415 
derms. The embryo adheres by the suckers at the ends of the 
tubes to a foreign body, and moves along by the contraction and 
expansion of the three feet. The peduncle is attached to the lower 
surface of the rudimentary star-fish, slightly excentrically and mid- 
way between two of the rays. The star-fish, though now only about 
once and a-half the size of the peduncle, has asserted distinctly its 
echinoderm character. The pentagonal form is distinctly recog- 
nisable, and plates of the characteristic calcified areolar tissue have 
begun to appear in the external layer, both on the upper and the 
under surface. These plates assume a definite arrangement ; and 
shortly a depression appears in the centre of the oral surface of the 
star-fish, and gradually deepens till it communicates with the central 
cavity of the body, forming the permanent mouth. Round the 
mouth a delicate tube forms an annular elevation, passing at one 
point under or into the base of the peduncle — the oesophageal cir- 
cular vessel of the ambulacral system ; and from this ring five deli- 
cate straight tubes pass to the ends of the rays, each tube flanked on 
either side by a row of rudimentary ambulacral suckers. 
Up to this period, the general granular contents of the embryo 
could be forced by slight pressure into the tubes of the peduncle. 
Now, however, the attachment of the peduncle to the embryo gradu- 
ally contracts, and at length it distinctly coalesces with, and becomes 
attached to, the annular ambulacral ring. Pressure applied to the 
peduncle no longer injects the general cavity of the star-fish ; it only 
dilates its ambulacral vascular system. The true ambulacral system 
of the embryo now develops rapidly, assuming its proper locomotive 
and respiratory functions, and the peduncle withers — the sucking 
feet first breaking off, and the body of the peduncle disappearing 
finally, as a gradually contracting sac hanging to the ambulacral 
ring, midway between two of the rays. At the point of disappear- 
ance no scar is afterwards perceptible. 
It would appear from the foregoing observations, that the first 
step in the development of this form of echinoderm embryo is the 
differentiation of a portion of the yolk into an investing layer of 
structureless “ Sarcode that the layer gradually increases in 
thickness ; and that finally, from one part of its surface, a branched 
peduncular process is produced, as an extension of the same trans- 
parent structureless material. The branches of this organ are ter- 
minated by suckers, and serve, among other functions, as organs of 
