238 E. W. MACBRIDE. 



circles, whilst the right hydrocoele and the right posterior 

 coelom remain small. 



(4) The gradual atrophy of the stalk. 



(5) The outgrowth of the adult oesophagus and the formation 

 of the new mouth on the left side. 



In the Crinoid the list would stand thus : 



(1) The constriction of the animal into calyx and stalk. 



(2) The displacement of the mouth and neighbouring organs, 

 i. e. the hydrocoele, to the posterior end of the body by unequal 

 growth. 



(3) The mutual displacement of the right and left posterior 

 coeloms, the left becoming posterior and the right anterior, 

 both having a ring-shaped growth. 



(4) The spiral growth of the intestine and formation of anus 

 close to primary madreporic pore. 



It will be seen that the Asterid metamorphosis is very 

 different from that of the Crinoid, being much simpler : one 

 great difference which strikes one at once being that in the 

 former case the ends of the hydrocoele grow so as to embrace 

 the stalk, which thus appears to spring from the oral surface ; 

 Avhereas in the latter case the hydrocoele is carried far away 

 from the stalk to the posterior end of the body. Much diligent 

 search has been made in the centre of the aboral surface of 

 Asterids for traces of a stalk, but to anyone who has grasped 

 the foregoing explanation it will be at once obvious how futile 

 such search must prove. PI. XXII, figs. 158 and 159, though 

 intended to indicate ancestral forms, illustrate the two metcV- 

 morphoses outlined above very well. 



The sections about to be described illustrating the meta- 

 morphosis are nearly all cut parallel to the larval plane, and 

 as was the case with the sections of the larva, where two or 

 three sections from the same series are figui'ed the most dorsal 

 is in every case placed first, and so one can clearly see their 

 relation to corresponding sections of the larva. As one always 

 thinks, however, of the organs of an Asterid as related to the 

 plane of the disc or adult plane, it will be well to repeat the 

 relation which these two planes bear to one another. The 



