ECHINODERMATA. 



215 



or rays one sees a small, round tubercle, which is persistent, and, 

 from its constancy and early manifestation, important : it becomes 

 the so-called eye. The back is now convex, and raised clear from 

 the arms. The quadrifid attaching pedicle still remains in the inter- 

 space between two of the arms, more towards the ventral than the 

 dorsal surface. 



On the 23rd of April the five arms were very distinct, the ambu- 

 lacral tentacles were longer, and now the young creeps by means of 

 them, and alters its position when turned on its back. The attaching 

 pedicle contracts, and becomes softer and more contractile. On May 

 the 1st it was reduced to two tubercles, and was more on the dorsal side, 

 at the angle between the two arms ; and the hitherto bilateral asteroid 

 now becomes the completely radiated animal ; the five arms lengthen, 

 their small spines grow, the ciliograde powers are lost, and the young 

 star-fishes creep away, and slowly acquire the mature dimensions. 



In the larva of another species of star-fish, to which larva Miiller * 

 gave the name of Bipinnaria, the long oblique fissure lodging the 

 mouth is bordered by a ciliated fringe, and there is a distinct circular 

 band of cilia above, and in front of the oral fissure, the anterior 

 boundary of which is formed by the hind part of that band. The 

 intestine is short and straight. The anus opens on the ventral surface 

 of the body. The Bipinnaria grows to the length of an inch or 

 more, chiefly by the increase of the anterior part of the body, long 

 processes being developed from the ciliated fringe and band, which 

 form two fin-like expansions one above the other. 



The next series of metamorphoses w^hich claim our attention are 

 those which Miiller has traced out in the genus Ophiura. In 1835, 

 this eminent physiologist and naturalist visited the coast at Heligoland, 

 accompanied by some of his pupils, and they occupied themselves in 

 investigating the minute forms of marine animal life. They met 

 with some singular forms, and the results of their researches were 

 communicated to the Academy of Sciences at Berlin. | Three minute 

 animals, about 1-4 or 1-2 line in diameter, were referred respectively 

 to the genera Mesotrocha, Vexillaria, and Pluteus. The first of these 

 afterwards proved to be the larva of a Nereis, the second of an 

 Ascidian, and the third of the Echinoderm in question. 



The Pluteus is compared, in form, to an easel ; it is better adapted 

 than the larvas of the Asterias for observation, because it is more 

 transparent. It resembles the larvae of the ordinary star-fishes in 

 having an elongated form, traversed by a straight intestine with the 

 mouth at one extremity and the anus near the other, and in being 

 girt by a ciliated fringe ; but the fringe passes above the mouth and 



* CLXXIII. p. 29. taf. vii., fig. 5—8. f CLXXI. 



p 4 



