ECHINUS. 325 
PLATE V. 
View of the interior of the test of Echinus escu- 
lentus. The three ambulacra shown are those 
forming the trivium. One gonad only is shown 
in situ. Nat. size. 
A blastula of Echinus, in optical section. x 210. 
A blastula passing into the gastrula stage, show- 
ing the amceboid mesenchyme cells. xX 210. 
Diagram showing the separation of the hydro- 
enteroccel from the archenteron. 
A fully-developed but not full-grown pluteus of 
Hchinus. X100. 
A pluteus, two days old, showing the beginning 
of the formation of the hydro-enteroceel. x 170 
A fully-formed gastrula, paseing into the pluteus 
stage. X 210. 
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