620 



ECHINODERMATA 



plates next appear as a ring of small ossicles between the orals 

 and basals, and simultaneously the arms make their appearance 

 as five outgrowths supported by the first radials, and by the other 

 radials when these appear. The free radial canals now become 

 adherent to the arms, but these canals soon give off paired 

 branches of unlimited growth, which are supported by bifurca- 

 tions of the primitive arms, and in this way the ten arms of the 

 adult are established. So far, then, as the water- vascular system 

 is concerned, the apparent forking is not a true dichotomy, but 

 results from the production of two opposite branches, whilst the 



Fig. 295. Four diagrams to explain the metamorphosis of the larva of Anted.on rosacea, 

 ax, Anterior coelom ; gen.st, genital stolon ; l.hy, left hydrocoel ; l.pc, left posterior 

 coelom ; mp, madreporic pore ; r.p.c. right posterior coelom ; stovi, stomodaeum. 

 (A, B, and C after Korschelt and Heider ; D after Perrier.) 



main axis ceases to grow. The appearance of cirri marks the 

 fusion of the uppermost stem-ossicles to form a centro-dorsal, 

 and shortly afterwards the young Antedon snaps off its stem and 

 swims away. 



Now in reviewing this life-history we cannot fail to be struck 

 with resemblances to the development of Asteroidea, and 

 especially to that of Asterina gibhosa. The absence of a con- 

 nexion between stomodaeum and gut is due to the embryonic 

 mode of life. On the other hand, the presence of a long prae- 

 oral lobe, containing an extension of the anterior coelom and 

 having a fixing organ at its apex, can only be paralleled among 

 Asteroidea. In broad outlines, then, up to the period of fixation 

 the two developments are parallel, but after this point a diverg- 



