EMBRYOLOGY. 



721 



peculiar to Brachiolaria. They are only capable of opening and shutting 

 like the rods of an umbrella. Fig. 56, which is Fig. 55 seen from above, when 

 left in its natural attitude, shows the extent to which the arms can be spread. 

 This does not prevent the pluteus from moving quite rapidly (a sort of glid- 



FlG. 56. 



Fig. 57. 



ing motion ), principally by means of the vibratile epaulettes, which perform 

 an important part in propelling the pluteus. While moving, the anal ex- 

 tremity is usually kept downward ; this is the position which has been given 

 to all the figures in this Memoir, and it is their natural position. Previous to 

 the time when the anal extremity is loaded down by the development of the 

 sea-urchin {Figs. 86-54), it is quite common to see them 

 moving in every possible attitude, so that it would be 

 difficult, from a knowledge of the earlier stages alone, 

 to ascertain with 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 pluteus. The embryos also assume during their 

 movements the oblique position described in Brachio- 

 laria ; this seems characteristic of all the Echinoderm 

 embryos I have had occasion to examine, whether 

 Ophiuran, Holothurian, Echinoid, or Asterian. A natu- 

 ral profile view of an adult pluteus (Fig. 57) cannot be 

 made with great accuracy, and the outline here given 

 is added simply to show the position of the arms ; ro- 

 tating as they do almost continually on their vertical 

 axis, we catch only passing glimpses of the exact pro- 

 file outline. The only adult pluteus figured in profile 

 by Miiller is found on Plate V. of his seventh Memoir. 



