268 THE PROTOCHORDATA. 



lost their sessile mode of existence, and have again become 

 free-living, retaining, however, their radial symmetry. At 

 the present time the fixed habit of life is only retained 

 by the members of one of the subdivisions of the Echino- 

 derm class ; namely, the Crinoidca. 



Most genera of Crinoids (RJiizocrinus, Pentacrinus, etc.) 

 remain fixed by a long, jointed stalk throughout life ; but 

 the well-known "feather-star," Antedon rosacea, is only 

 fixed during a certain period of its larval development. At 

 the close of the period of fixation the body of the animal, 

 or, as it is called, the calyx, breaks away from the stalk by 

 which it was attached to the rocks, and so begins to lead a 

 free existence, being capable of swimming vigorously by 

 the flapping of its arms. 



Although the existing Crinoids have become extensively 

 modified along their particular line of evolution, yet there 

 is reason to believe that they represent the more im- 

 mediate descendants of the primaeval form which ex- 

 changed its primitively free life and bilateral symmetry for 

 a sessile existence and radial symmetry. This view is 

 strengthened by the character of the free-swimming larva 

 of Antedon. This larva does not possess, in any extrava- 

 gant degree, those fantastic structures which are so 

 characteristic of other Echinoderm larvae, such as the 

 provisional ciliated processes or arms of the " Pluteus " 

 (larva of sea-urchins), or the undulating ciliated bands of 

 Auricularia. 



On the contrary, the larva of Antedon is a simple 

 barrel-shaped organism, with regular ciliated bands pass- 

 ing around it (Fig. 128). 



Perhaps the structure which, above all, stamps the free- 

 swimming larva of Antedon as having, from a phylogenetic 

 point of view, a more primitive type of organisation than 



