722 



EMBKYOLOGY. 



An adult pluteus, in the condition of Fig. 55, requires several weeks for 

 the complete development of the Echinus and the resorption of the plutean 



framework. The Echinus encroaches gradually on 

 the anal extremity ; the hase of the arms / and e" 

 is soon lost in the midst of the spines of the young 

 sea-urchin, which are arranged in a conical, open, 

 spiral wreath, surrounding the mouth [Fig. 58). 

 While 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' v , 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 lias lost entirety its former symmetrical appearance 



{Fig. 58). 



The figures given by Midler on Plate 111. of his first Memoir* represent 

 several Echinoid embryos in which the young Echinus has resorbed more or 

 less of the plutean frame. From what I base observed on several of these 

 embryos, the pluteus is as completely resorbed as is the case in the Brachiolaria 

 observed by me. Not a single part of the framework is thrown oil'; the pro- 

 cess of resorption begins at the base of the arms; they are thus gradually 

 shortened, the rods apparently melt away before our eyes, the extremity of 

 the arms is the last to disappear; and immediately before the time when the 

 young Echinus is freed from the plutean appendages, the extremities of all 

 the arms are still there, as perfect as when these appendages stretched sym- 

 metrically on both sides of the longitudinal axis. From many of the figures 

 of Midler himself it is evident that, in the embryos he has observed, the young 

 Echinus resorbs the whole of the framework, and does not separate from it by 

 losing the arms, as he has stated. (See Plates III., IV .. V.. VI. of his first and 

 Plate VIII. of his seventh Memoir.) The pluteus represented in Fig. 55 was 

 kept in confinement from the 1st of October to the 20th of November before 

 every trace of the arms bad disappeared, when the young sea-urchin had 

 assumed the appearance of PL IX. f. 1. This was drawn from a specimen 

 found floating on the surface in the middle of June. This young sea-urchin 

 bears a striking resemblance to a young Arbacia figured by Miiller on Plate IV. 



* Muller, J., L'eber die Larven und die Metamorphose der Ophiuren und Seeigel. Berlin, 1848. 



