ON THE EMBRYOLOGY OF ECIIINODERMS. 9 



The presence of rods in the plutean forms of Ophiurans and Echinoids of course 

 restricts considerably the play of the arms in assisting the motion of the larva. The 

 arms cannot be bent and twisted in the graceful manner so peculiar to Brachiolaria. 

 They are only capable of opening and shutting like the rods of an umbrella. Fig. 

 21, which is fig. 20 seen from above, when left in its natural attitude, shows the extent 

 to which the arms can be spread. This docs not prevent the larva from moving quite 

 rapidly by means of a kind of gliding motion, in which the vibratile epaulettes perform 

 an important part in propelling the Pluteus. While moving, the anal extremity is 

 usually kept below, as in the position which has been given to all the figures in this 

 Memoir, which is their natural position. Previous to the time when the anal extremity 

 is loaded down by the presence of the Sea-urchin (figs. 1-19), it is quite common to 

 see them moving in every possible direction, so that it would be difficult, from a knowl- 

 edge of the earlier stages alone, to ascertain Avith precision what the natural attitude 

 is ; although we notice even in the early periods a very strong tendency to assume the 

 natural position of the adult larva. The larvae also assume during their movements 

 the oblique position described in Brachiolaria ; this seems characteristic of all the 

 Echinoderm larvae I have had occasion to examine, whether Ophiuran, Holothurian, 

 Echinoid, or Asterian. A natural profile view of an adult larva (fig. 22) cannot be 

 made with great accuracy, and the outline here given is added simply to show the 

 position of the arms ; rotating as they do almost continually on their vertical axis, we 

 catch only passing glimpses of the exact profile outline. The only adult larva figured 

 in profile by Miiller is on Plate V. of his seventh Memoir. 



An adult Pluteus, in the condition of fig. 20, requires several weeks for the com- 

 pletion of the Echinus and the absorption of the plutean framework. The Echinus 

 encroaches gradually on the anal extremity ; the base of the arms, e' and e", is soon lost 

 in the midst of the spines of the young Sea-urchin, which are arranged in a conical 

 open spiral Avreath surrounding the mouth, fig. 23. \Yhile this encroachment of the 

 anal extremity is going on, the oesophagus has contracted to such an extent that the 

 base of the oral arms, e", e'", is brought directly in contact with the anal vibratile chord. 

 During the process of resorption the arms have lost their mobility ; they appear like 

 helpless rods, stretching at every conceivable angle from the Pluteus, which has lost 

 entirely its former symmetrical appearance, fig. 23. 



The young Echinus after the Resorption of the Pluteus. 



The figures given by Miiller on Plate III. of his first Memoir * represent several 



• MiJLLEi^ J. Ueber die Larven und die Metamorphose dcr Ophiuren und Seeigel. Berlin, 1848. 

 VOL. IX. 2 



