ECUINODERMATA. 217 



atti'acted by the circumstance of some of the minute Plutei differing 

 from others in certain proportions, in having three processes with cal- 

 careous rods developed from the hinder convexity of the larva, and 

 by the development of particular tubercles, richly beset with cilia. 



V. Baer, who led the way in experimenting and observing the 

 artificially impregnated eggs of the Sea-urchins {Echinus), found 

 them to be first developed into a ciliated monadiform animalcule, like 

 that of tlie Medusa atirita. In the course of four days this had be 

 come metamorphosed into a minute transparent acalephoid creature 

 w^hich he compares to a Beroe. Here his observations ceased, the 

 animals having perished on the fifth day. 



In the autumn of 1845 Professor Miiller discovered in the sea at 

 Heligoland the transparent acalephoid animalcules which have been 

 mentioned, referable by this mode of motion to the "ciliograde" 

 order, one of which, from its singular form, somewhat resembling a 

 painter's easel, he described under the name of Pluteus paradoxus. 

 Returning in the following year to the same field of observation, 

 he pursued with admirable detail the change of this species, as 

 above described, into an Ophiura (Brittle star-fish), and that of the 

 other kind of Pluteus into an Echinus. 



The larva of the Echiims, like that of the Ophiura, has a quadri- 

 lateral pyramidal body, of a colourless, transparent, or hyaline sub- 

 stance, dome-shaped behind, expanded and slightly excavated in 

 front, the corners of which are prolonged into straight slender legs, 

 strengthened by filiform rods of calcareous matter, reaching to the 

 summit of the dome : the mouth is prolonged from the middle of the 

 concave base into a four-sided proboscis, the angles of which are also 

 produced into slender processes shorter than the four outer legs. 

 "Were the creature to stand upon these it would resemble a table-clock 

 case, and the proboscidiform mouth and appendages might swing to 

 and fro like the pendulum. The larva is, however, a free- swimming 

 animalcule, and propels itself, with the base and processes forwards, 

 chiefly by means of strong cilia, grouped on two tubercles at the 

 sides of the dome, which are compared to epaulets : but it is also 

 provided with a fringe of cilia, girting the dome and continued upon 

 all the columns, up one side and down the othei\ The mouth is sur- 

 rounded by a distinct band of cilia : it is triangular and bounded 

 inferiorly by an oblique projecting lip. In the centime of the dome- 

 shaped body is a subspherical granular mass, apparently the remnant 

 of the primordial germ-mass, which Professor Miiller calls the 

 stomach, and which doubtless surrounds the cavity to which the 

 mouth and pharynx conduct. 



The first evidence of the change to the Echinus is the formation 



