xvi ECHINODERMATA 503 



presence of yolk, and so we are the less surprised at its abnormality in 

 Ophiura. But it should be added that Grave had only a limited 

 amount of material to work on, which he could not augment as 

 second attempts at artificial fertilization did not succeed. Hence it 

 is possible that some of his interpretations may be mistaken. 



DEVELOPMENT OF OFHIOTHRIX AND ASTERIAS COMPARED 



When we compare the development of OpMothrix with that of 

 Asterias we notice two main differences : first, the fixed stage is dropped 

 out completely ; the Ophiuroid, by the retention of the postero-lateral 

 larval arms, is enabled to pass through its metamorphosis floating ; 

 and, secondly, some of the larval changes are simpler and more direct 

 in character than in Asterias. Thus the segmentation of the coelom 

 is not complicated by subsequent perforations of the septum, and the 

 larval mouth persists. The perforations of the septum seem to be 

 due to the body musculature developed in Bipinnaria, which renders 

 it necessary to allow the coelomic fluid to pass from one part to 

 another. It is hardly likely that, in the primitive condition of 

 affairs, if a segmentation took place it would be largely undone 

 immediately afterwards, so that in this respect Opliiothrix is prob- 

 ably more primitive than Asterias. 



Then, as to the retention of the larval mouth in the adult, there 

 can be no question that this more truly represents the history of the 

 race than the obliteration of the old mouth and the formation of a 

 new one in a new place. It is not easy to picture an animal in the 

 condition of losing an old mouth and gaining a new one. We have 

 seen that when the hydrocoele ring is just complete there is no epi- 

 neural fold developed, so that the nerve cord is exposed ; and further, 

 that when metamorphosis is just complete the animal walks on its 

 tube feet instead of using its arms. Young specimens of Amphiura 

 squamata, if cut out of the bursa of the parent when the arms are as 

 yet undeveloped, walk also on their tube feet, so that in these two 

 respects the Ophiuroid may be said to pass through an Asteroid stage. 



Now there are found in Silurian and Devonian rocks a consider- 

 able number of fossils exactly intermediate in character between 

 Asteroidea and Ophiuroidea. We have every reason to believe that 

 in these fossils we possess the actual record of the evolution of the 

 Ophiuroidea, and we are therefore in a position to test how far 

 the history of the individual, as disclosed by embryology, agrees with 

 the history of the race. Now these fossils show an open ambulacral 

 groove, and ambulacral ossicles not yet united to form vertebrae, but 

 in the larva the open ambulacral groove becomes closed long before 

 there is any trace of vertebrae, and while the adult arms are still 

 mere stumps. 



Hence we conclude that in the larval history there has been a 

 dislocation of the sequence of events, and that the formation of the 

 epineural flaps has been hurried on long before its time in a word, 



