ILLUSTRATIONS. xvii 



PLATE X. OPPOSITE PAGE 152. 

 THE ECHINUS AND ITS LARVA THE PLUTEUS. 



1st. The Pluteus or nurse form of the Echinoids, magnified 

 20 diameters. 



2nd. The Echinus, the spiny sea-egg or sea-urchin, reduced 

 to \ natural size. 



The Echinus which is here represented is found in holes that 

 it has drilled near low tide in the surf-beaten rocks of 

 the Bermuda coast. From its eggs is produced the 

 Pluteus, an intermediate generation, almost microscopic 

 in size, but an active swimmer by means of its numer- 

 ous cilia-hairs. It is an entirely different animal from 

 its parent, not having one point of structure in com- 

 mon with it. When this creature attains its minute 

 growth a tiny saucer-shaped disk appears on one side of 

 the stomach of the Pluteus, gradually growing at the 

 edges into a globular form until it has enclosed the vital 

 parts of its old nurse. It then forms a new mouth at 

 the point last enclosed, discards all the external parts of 

 the Pluteus, and sinks to the bottom of the ocean to 

 begin the new life of the spiny Echinus. Page 124 



PLATE XL OPPOSITE PAGE 168. 

 TRANSVERSE SECTIONS OF SPINES OF ECHINUS. 



Magnified about 30 diameters, making the largest section 

 one-twelfth of an inch across. There is an infinite var- 

 iety in the beauty, the structure, and the exquisite 

 coloring of these thin sections. Pages 122-3 



By request is here published the author's method of cutting 

 and grinding these sections in quantity: Imbed the 

 spines in glue, making a stick of them. When nearly 

 solid with glue and dry, cut into thin cross sections with 

 a dentist's saw. Dissolve out the glue in hot water. 

 Dry the sections and imbed them in Canada balsam on 

 two pieces of glass. When hardened by heat, grind one 

 against the other with pumice and water until level and 

 smooth. Place another piece of glass in a saucer and 

 cover with turpentine. Put one of the ground pieces 

 on this with sections down. After one day the sections 

 w r ill all lie turned over on the lower glass. Carefully 

 raise and imbed them as they lie in Canada balsam. 



