112 



HANDBOOK OF INVERTEBRATE ZOOLOGY. 



view in Fig. 64, has undergone very considerable changes, 

 and is now sufficiently transparent to allow the internal 

 organs to be more minutely examined. 



The ends of the ciliated ridge 

 have grown forward so as to form 

 a pair of ear-like processes (Figs. 

 63 and 64, c, c) , the rudiments of 

 the pair of post-oral arms. The 

 cells of the ridge have become 

 thickened, columnar, and very dif- 

 ferent in appearance from the ordi- 

 nary ectoderm cells. They carry 

 long cilia, and are arranged in a 

 row which runs out to the tips of 

 the arms, and after bending around 

 them, turns towards the dorsal sur- 

 face, and bending forward, runs 

 along the free edge of the oral 

 lobe (a). 



Great changes have also taken 



place in the spicular skeleton, 



which is now quite well developed. 



The rods (cZ), which run into the 



posterior lobe, and which we may 



Fig. 64. — Side view of call the lateral spicules, nearly 



thesamelarva^(Drawnfrom^^^^ each other upon the median 



nature by W. K. Brooks. ) ^ . 



line, and their free posterior ends 



have enlarged into irregular, club-shaped masses. The 

 two branches which, at an earlier stage, ran towards the 

 middle of the ciliated ridge, have met and united so as to 

 form a solid bar («^, which may be called the ventral 

 transverse rod, and which crosses the ventral surface. 

 The branches which, at an earlier stage, ran towards the 



Fig. 64. 



