160 SYNOPTIC COLLECTION. 



Antedon passes through the condition already shown by 

 Millericrinus (No. 281). 



The armless Cystoid stage passes into the Penta- 

 crinoid stage, in which uniserial arms grow out (fig. 3). 

 These continue to increase in number. The centro-dorsal 

 plate develops cirri, and by this time all trace of the 

 underbasals is lost. 1 



When ten cirri are developed,, a separation takes place 

 between the centro-dorsal plate and the stem ; the latter 

 is left attached, while the animal breaks away and is 

 henceforth free. 



A tiny opening is left in the middle of the aboral side, 

 but later this is filled, though traces of the scar may be 

 seen internally. 



After the animal liberates itself from its stem, it swims 

 freely in the water. The cirri are used occasionally for 

 crawling about on marine plants, or at other times for 

 anchoring itself to rocks. According to Carpenter, 2 the 

 adult Antedon has the habit of fixing itself to a rock and 

 remaining for long periods. In this stage the body is 

 extremely small and the basals have become metamor- 

 phosed into the so called rosette, which is wholly con- 

 cealed in the cavity of the ring formed by the radials. 



The mouth is in the middle of the oral disc and at one 

 side of this opening is the anal tube. The five arms 

 divide almost immediately to form ten organs which are 

 disproportionately large for the size of the body and are 

 well provided with pinnules. 



The floor of each ambulacral groove is ciliated, and the 

 five ciliated grooves extend from the mouth over the oral 

 disc to the ends of the arms. 



Actinometra (No. 289) is similar to Antedon but differs 

 from it by having the mouth at one side of the oral disc 

 and the anal tube near the center. 



1 Carpenter, stated by Wachsmuth and Springer, Proc. Acad. 

 Nat. Sci. Phila., 1888, p. 352. 



2 Phil. Trans. Roy. Soc. London, CLVI, 1866, p. 698. 



