THK DEVELOPMENT OP OPHIOTHRIX FRAGILIS. 585 



the case ; the process as repeated iu ontogeny is pushed back 

 to an earlier stage iu development than that at which (to 

 judge from palteoutology) it actually occurred in the history 

 of the race. 



When by the meeting of the epineural flaps the epineural 

 canals are formed, and when at the same time the hydroccele 

 ring is completed by the meeting of its dorsal and ventral 

 ends, the second stage in the metamorphosis may be said to 

 be reached. Its external appearance is shown in fig. 15, which 

 represents a larva twenty-three days old raised from artificial 

 fertilisation in 1898. It will be observed that the left antero- 

 lateral arm has been carried over the oesophagus until it has 

 come into close contact with its right fellow, by the double 

 process of the preponderant growth of the organs of the left 

 side, and the shrinkage of the thin-walled forehead of the 

 larva which intervenes between the two antero-lateral arms. 

 This forehead is the very much reduced representative of the 

 prse-oral lobe of the Asterina gibbosa larva, and its shrink- 

 age corresponds to the atrophy of the prse-oral lobe or stalk 

 of Asteroidea. At the same time, all the larval arms except 

 the postero-lateral become reduced in size. The two horns of 

 the left posterior coelom meet one another, and thus the arm 

 rudiments are brought into juxtaposition with the hydroccele 

 lobes. This is, however, effected in such a way that arm 

 rudiment 1 comes to rest on hydroccele lobe 2, whilst 

 hydroccele lobe No. 1, which has been carried over the 

 oesophagus, is supported by arm rudiment 5, which is an out- 

 growth from that horn of the left posterior coelom, which has 

 passed under the oesophagus. Here, again, the larva of 

 Ophiothrix fragilis agrees with the larva of Asterina 

 gibbosa. 



Each hydroccele lobe has given rise to two or three pairs 

 of lateral lobes which are the rudiments of the first paired 

 tentacles, the original lobe giving rise to the terminal or 

 azygous tentacle of each arm. New tentacles are formed by 

 acropetal buds from the median tentacle. The perihtemal 

 cavities shut off from the left posterior coelom have expanded 



