126 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



surface, and hangs over the mouth. The anterior edge 

 of this lip is prolonged into two pairs of secondary oral 

 arms (a" and a'"), those nearest the middle line being 

 much the smallest. 



The median dorsal spicule (u) which appeared at the 

 last stage, has now lengthened, to form a large U, which 

 lies on the dorsal surface, and sends a branch into each of 

 the outer secondary oral lobes. 



On the ventral surface of the body (Fig. 72), the ciliated 

 ridge has grown backwards on each side, between the 

 post-oral lobe and the bases of the post-oral arms, to 

 form a pair of ear-like processes (au'), which are fringed 

 with cilia. 



On the dorsal surface (Fig. 71) a similar pair of ear- 

 like processes (au) have been formed l)y the development 

 and folding of two lines of ciliated cells, one on each 

 side of, and parallel to the middle line of the body. 



At this stage, the stomach is slightly pushed to one 

 side by the development of five hollow tubes (Figs. 71 

 and 72, ab), on one side of it. These are the first five 

 tubular ambulacra of the young sea-urchin, and they are 

 on the right side of the stomach in a dorsal, on the left in 

 a ventral view. 



In the star-fish larva, where their origin can be more 

 satisfactorily studied, it will be seen that they are devel- 

 oped from the left water-tube, and not from the actual 

 walls of the stomach. 



^. The development of the young sea-urchin. 



As the development of the young echinoderm within 

 the larva can be studied to more advantage in the star-fish 

 than in the sea-urchin, its formation will be more fully 

 described under that heading, but the following points 

 should be noticed in the pluteus of Arbacia. 



