METAZOA ECHINODERMA. 1 59 



The large branching arms with their many pinnules are 

 extremely graceful organs. The disc is small and the 

 ambulacra extend from the mouth to the ends of the arms. 



The body is also small in Pentacrinus (No. 285), con- 

 sisting chiefly of five basals and five radials. There are 

 vestiges of underbasals, but these are sometimes wholly 

 resorbed in the adult. Pentacrinus is attached by a long 

 pentagonal stem (No. 286), the joints of which bear cirri. 

 The ventral surface is flexible and has many irregular 

 plates. The mouth is exposed and from it extend the five 

 uncovered ambulacra. 



The uniserial arms are greatly developed, having a 

 large number of branches which are well supplied with 

 pinnules. 



Antedon ( = Comatuld) rosacea Linck (PL 287 ; No. 288) 

 is a living Crinoid of great interest, inasmuch as its devel- 

 opment recapitulates in a marked degree the history of 

 the group to which it belongs. 



After escaping from the egg, the embryo is free and 

 moves by means of bands of cilia (PI. 287, fig. i). Early 

 on the eighth day (Bury) after development began, the 

 larva became attached. On the tenth day the larva had 

 developed a stem (fig. 2). In this stage the underbasals 

 are found (fig. 2. ub} and above these the basals (fig. 2, 

 b) . Although the sutures indicate only three under- 

 basal plates, one small and two large, nevertheless it is 

 probable that each large plate is formed by the coales- 

 cence of two plates, so that originally there were five 

 underbasals. 1 



The resemblance of Antedon to a Cystoid is now strik- 

 ing, and the existence of underbasals in the young is evi- 

 dence of descent from the Palaeozoic Crinoids in which 

 we have already seen underbasals well developed. As 

 Antedon grows older, the underbasals and the top stem 

 joint fuse into a single plate, the centro-dorsal, so that 



'Phil. Trans. Roy. Soc. London, CLXXIX, 1888, p. 288. 



