THE DEVELOPMENT OF OPHIOTURIX EUAGILIS. 597 



Ecliiiioidea, viz. iuto three somites on each side^ but the 

 middle somite on the right side is not shifted dorsally as it 

 is in Asteroidea and Echiuoidea. This somite occasionally 

 assumes a five-lobed form, proving beyond doubt that it is a 

 right antimere of the water-vascular system. 



(4) The left hydrocoele is budded from the anterior division 

 of the left coelom, not from the posterior division as Bury 

 supposed (5), and its persistent connection with the left 

 anterior coelom constitutes the stone-canal; this opens into 

 the hydrocoele between lobes 1 and 2, as in Asteroidea, not 

 between lobes 4 and 5 as Bury asserted. 



(5) The metamorphosis is initiated by a preponderant 

 growth of the organs of the left side, which affects the 

 larval arms and the sides of the oesophagus, and which not 

 only carries the hydrocoele round the oesophagus but also the 

 madreporic pore and the left anterior coelom, so that these 

 come to be near the right hydrocoele, 



(6) The periha3mal system of canals originates as a series 

 of five hollow evaginations, the cavities being small and the 

 walls thick. The first originates from the left anterior 

 coelom, the other four from the left posterior coelom. From 

 their walls originate the motor ganglion cells and in all 

 probability the ventral inter-vertebral muscles. 



(7) The left posterior coelom gives rise to a dorsal and a 

 ventral horn, which eventually meet, causing it to assume a 

 ring-shape. From it five evaginations give rise to the arm 

 rudiments and the first of these comes to overlie hydrocoele 

 lobe No. 2. 



(8) A peri-oral coelom closely surrounding the adult 

 oesophagus originates from the left posterior coelom. 



(9) A series of epineural ridges, the tops of which grow 

 out so as to form arch-like folds, are formed inter-radially 

 alternating with the primary tentacles. By the union of 

 adjacent arches the basal portions of the tentacles are 

 covered, exactly as occurs in the Echinoid larva. 



(10) The adult oesophagus is formed from the left inner 

 portion of the larval one: its covering is made of the adoral 



