558 E. W. MACBRIDE. 



wider range of students than specialists in the class Ecliino- 

 dermata. The most interesting result wliicli was obtained 

 from the study of the development of Asteroidea and 

 Echinoidea was the discovery that the ca3lom of the larva 

 showed distinct traces of metameric segmentation, a division 

 into three somites being clearly indicated. The pioneer in 

 this work is Bury, to whose stimulating papers (4, 5, find 6) 

 I wish on this occasion, as formerly, to express my deep 

 indebtedness. Buiy, however, only recognised two somites, 

 whilst the main point of my previous researches was that 

 the hydrocoele, or rudiment of the water-vascular system, 

 which Bury regarded as essentially a single organ, was 

 in reality paired, the left rudiment developing whilst the 

 right remained small and insignificant. This interpretation 

 of the facts has been challenged by other observers such as 

 Goto (11) and Masterman (20), who interpret what I regard 

 as the right hydrocoele in a different manner. I may summa- 

 rise the results of the present paper by saying that the study 

 of the development of Ophiuroidea has afforded me a com- 

 plete confirmation of my views. The state of affairs in the 

 Ophiuroidea is much clearer and simpler than in the other 

 classes of Echinodermata, and I hope to convince every 

 unprejudiced reader of this paper that the Echinoderm larva 

 really possesses three metameres. 



HiSTOEiCAL Sketch. 



Our knowledge of the development of Ophiuroidea dates 

 back to Johannes Miiller. At the meeting of the Academy 

 of Sciences of Berlin, held on December 4th, 1845, this 

 celebrated naturalist described a number of new marine 

 animals which ho had observed whilst at Heligoland in the 

 autumn of that year. Amongst these was one which Miiller 

 named Piute us paradoxus, from its fancied resemblance 

 to a painter's easel. This description was pnblislied in the 

 ' Arcliiv fiir Anatomic und Physiologic,^ in 1840 (24), and 

 on October 29th of that year Miiller read before the Academy 



