ANELLATA. 129 



The small Pentacrini entirely disappear in September, at which 

 season the young Comatulae make their appearance. It is the opinion 

 of Mr. Thompson, the discoverer of the Pentacrinus Europceus, that 

 this pedunculated star-fish is a transitional state of the young Comatula, 

 an opinion which is adopted by Mr. Thompson, Mr. Ball, and Mr. Forbes, 

 experienced naturalists, who have each obtained and compared the Pen- 

 tacrini and young Comatulae. The actual metamorphosis of the Pen- 

 tacrinus into the Comatula has not yet been seen : we must suppose 

 that it enters life at first in the active stage of a ciliated gemmule ; that 

 it next selects the appropriate situation for its sedentary pentacrinite 

 stage of existence, and, finally dropping from the stalk, by an act of 

 transverse fission, a second time assumes a free condition of existence 

 under its mature form. Nor are these metamorphoses a whit more ex- 

 traordinary than those of the gelatinous Medusae : nay, the parallel 

 would be extremely close, since we saw that the Cyancea entered life 

 as a ciliated locomotive infusory, then became a sedentary polype, 

 supported on a central stem, which, finally, resolved itself into the 

 freely swimming Acalephans by several transverse fissions. 



Other highly interesting considerations arise out of the pre- 

 dominance of the Pentacrinite forms over the Asteriae or Echini, in 

 the limestones of the ancient transition epoch in Geology. As we 

 advance in our survey of the organisation and metamorphoses of 

 animals, we shall meet with many examples, in which the embryonic 

 forms and conditions of structure of existing species have, at former 

 periods, been persistent and common, and represented by mature and 

 p recreative species, sometimes upon a gigantic scale. 



LECTUEE XL 



ANELLATA. 



In both the Infusorial and Entozoic classes the body assumes a 

 more perfect linear and bilateral form as the species advance in the 

 scale of organisation ; and we have seen in the subjects of the pre- 

 ceding discourse, that even the typical radiated class of the zoophytic 

 sub-kingdom conducts by the Holothurian and Sipuncular families to 

 the vermiform type of the articulated sub-kingdom, in which the ve- 

 getative principle of development, by the frequent repetition of 

 similar parts, is still conspicuously manifested, but exercises its 



K 



